Live Updates: Biden Seeks to Reassure Democrats After Shaky Debate

The pledges of support for Mr. Biden after the debate stood in contrast to the alarm expressed early and openly in the media and among political strategists, even by some of Mr. Biden’s longest standing supporters, in the immediate wake of Thursday night’s debate. Joe Scarborough, the MSNBC host and a stalwart Biden defender, said questions about Mr. Biden’s fitness to run were now unavoidable. Opinion writers and liberal pundits called for him to step aside, albeit while expressing admiration for his record.

The hand-wringing began minutes into the debate, as Mr. Biden appeared meandering and unsteady in his face-off with former President Donald J. Trump. The debate devolved into an ugly performance of animosity and falsehoods, as a blustery Mr. Trump and Mr. Biden mixed petty personal attacks with strikingly different diagnoses of the nation’s biggest issues.

Mr. Biden, 81, and Mr. Trump, 78, spent their 90 minutes on stage in Atlanta debating inflation and immigration, abortion and addiction. Yet the extraordinary rematch between two presidents — two men who are the oldest candidates to ever seek the White House and who did nothing to conceal their hatred for each other — put on stark display the reasons the contest has repelled swaths of Americans.

Mr. Biden stumbled over his words, often pausing to correct phrases midsentence, sometimes winding to an uncertain finish. His voice was hoarse, which his campaign later attributed to a cold. He flubbed key figures. While he gained steam over the course of the debate, insulting Mr. Trump with greater vigor, his energy might have come too late for many viewers.

After the debate, Mr. Biden told reporters outside a Waffle House that he thought he “did well.” When asked about calls for him to drop out, he said: “It’s hard to debate a liar.” On Friday, top campaign officials plan to meet with donors in Atlanta, where they will probably face some difficult questions about the path ahead.

Mr. Trump made wild accusations riddled with falsehoods, portraying a nation under violent siege from undocumented immigrants — a claim unsupported by broader crime statistics — as well as grandiose exaggerations. The format of the debate, in which the candidates were blocked from interrupting each other, appeared to serve Mr. Trump, who often appeared content to sit back and let Mr. Biden struggle.

Here’s what else to know:

• Democratic cleanup: The Biden campaign moved quickly to recast the debate, releasing memos and statements arguing that the event moved undecided voters to their side. Vice President Kamala Harris, in an interview on CNN, acknowledged a “slow start.” But, she said, Mr. Biden has no plans to step aside. “It was a strong finish,” she said.

• Next stops: On Friday, Mr. Biden will travel to Raleigh, N.C., for a rally with the first lady in a more conservative state that some Democrats believe could trend in their favor. Later in the day, he plans to be in New York City for a fund-raiser. Mr. Trump will head to an event in Chesapeake, Va., an effort to showcase new support in a state that has rejected him twice.

• Tough words: The shifted format of the debate, hosted by CNN, was designed to create a more civil discourse. But things quickly took an uncivil turn. Mr. Trump called Mr. Biden a “Manchurian candidate.” Mr. Biden accused his predecessor of having “sex with a porn star” while his wife was pregnant. The two men bickered over their golf scores and cognitive tests.

• Not just a number: When asked directly about his age, Mr. Biden began by noting he had been the second-youngest person elected to the Senate more than a half-century ago. Then he segued into a winding answer about boosting semiconductor-chip manufacturing and American greatness. Mr. Trump answered by saying he had “aced” cognitive tests. “I feel that I’m in as good a shape as I was 25, 30 years ago. In fact, I’m probably a little lighter.” Then he insulted Mr. Biden’s golf scores. The president took the bait, claiming he had a six — no, an eight — handicap. “Let’s not act like children,” said Mr. Trump, ending the discussion.

• Biden meanders: Mr. Biden appeared to lose his train of thought on an early question about national debt, concluding an answer that drifted from taxes to child care to health care to the word “Covid” before saying, incongruously, that he “finally beat Medicare.” Mr. Trump jumped on the misstep, accusing Mr. Biden of “wiping out” Social Security and Medicare. A few minutes later, after Mr. Biden trailed off again, Mr. Trump pounced. “I really don’t know what he said at the end of that sentence. I don’t think he knows what he said, either,” Mr. Trump said.

• Gaining momentum: Over the course of the face-off, Mr. Biden’s answers grew stronger. He offered some tough attacks on Mr. Trump’s indictments. “The only person in this stage that is a convicted felon is the man I’m looking at right now,” he said. Mr. Trump was defiant about his actions on Jan. 6, 2021, attempting in his remarks to transform the violent siege on the Capitol into a minor disruption. “On Jan. 6, we were respected all over the world, all over the world we were respected. And then he comes in and we’re now laughed at,” Mr. Trump said.

• Abortion: Mr. Biden fumbled on abortion rights, an issue that’s widely viewed as one of the strongest for Democrats. He struggled to explain Roe v. Wade and promised to “restore” the decision, a vow that will be almost impossible to keep even if Democrats win strong margins in Congress.

But.

“He spent much of the night with his mouth agape and his eyes darting back and forth,” the anchor said. “He couldn’t fact-check anything Donald Trump said. He missed one layup after another after another.” Now, he concluded, “is the last chance for Democrats to decide whether this man we’ve known and loved for a very long time is up to the task of running for president of the United States.”

This was no mere act of punditry. Mr. Biden is a skeptic of the news media, but Mr. Scarborough is among a tiny group of commentators that actually has his ear. The president has spent time with the anchor and regularly watches “Morning Joe,” a show that has defended him against all manner of attacks.

No more. And Mr. Scarborough was not alone. His defection mirrored that of other longtime Biden media allies who, often in elegiac and pained tones, urged the president on Friday to drop out after his shaky performance in Thursday’s debate against former president Donald J. Trump.

Thomas L. Friedman, a columnist for The New York Times and a regular visitor to Mr. Biden’s White House, wrote that he had wept watching the president. “Joe Biden, a good man and a good president, has no business running for re-election,” he said. Evan Osnos, Mr. Biden’s biographer and one of the few journalists granted extensive access to the president, said on CNN that the president had appeared “diminished.”

And on CNN, the Democratic analyst Van Jones delivered a soliloquy brimming with emotion, full of poignancy, defiance and regret.

“I just want to speak from my heart,” Mr. Jones said minutes after the debate ended. “He’s a good man. He loves his country. He’s doing the best that he can. But he had a test to meet tonight to restore the confidence of the country and of the base, and he failed to do that.” Mr. Jones paused for a breath. “There is time for this party to figure out a different way forward.”

Speaker Mike Johnson commented on an effort by a Republican congressman to invoke the 25th Amendment, which allows the vice president and members of the cabinet to discharge the president of his duties and powers if they no longer deem him capable of performing the job.

“It’s the cabinet that makes that decision,” he said. “I would ask the cabinet members to search their hearts. You’ve all been reporting all day long about the panic that has ensued in the Democratic Party. I would be panicking too if I were a Democrat today and that was my nominee.”

Two days ago, former Representative Adam Kinzinger, a Republican and a Trump critic, was in Atlanta to endorse President Biden. After Biden’s shaky debate performance, he said Democrats now need to do whatever gives them the best chance of defeating Trump.

“The big challenge Joe Biden had was the age question, and he certainly didn’t overcome that,” Kinzinger told The New York Times. “My goal is to defeat Donald Trump. He is an existential threat. Democrats have to make a decision about what is their strongest hand to do that.”

Environmental activists jumped on the omission.

“Donald Trump’s response to tonight’s sole climate question was a crash course in his real-life approach to the climate crisis,” said Lena Moffitt, the executive director of Evergreen Action, a climate advocacy group. “First, he ignored it entirely to talk about something else, and when that stopped working, he started lying.”

Mr. Trump did boast about what he said his administration had accomplished, including that “we had H2O, we had the best.” It was an apparent reference to his claim that the United States had the “cleanest” air and water during his presidency. (That claim is untrue. The United States ranked 16th in air quality and 26th in water sanitation in the world during his term, according to the Environmental Performance Index, a joint project from Yale and Columbia universities that ranks countries by a variety of environmental indicators.)

The former president also said that the United States was “energy-independent” on Jan. 6, 2021, the day his supporters stormed the Capitol. Mr. Trump frequently claims that he achieved energy independence and suggests that is not true under Mr. Biden.

The United States has been producing record amounts of oil under the Biden administration, according to the U.S. Energy Information Administration. During Mr. Biden’s term, the country has exported more energy — including petroleum — than it has imported, and it has produced more energy than it has consumed, the data shows.

Mr. Biden said that his administration had ushered in the “most extensive climate change legislation in history,” which is accurate. The Inflation Reduction Act, which he signed in 2022, invests $370 billion into renewable energy over the next 10 years.

Miguel Lorenzo, 34, a Mexican lawyer, said he wished the candidates had talked about the economic relationship between the countries, like the United States-Mexico-Canada Agreement or “near-shoring.” In February, Mexico outpaced China for the first time in 20 years to become the United States’ top source of official imports.

“Maybe they’re not focusing on it over there because they don’t have as much interest as us here,” said Mr. Lorenzo, who met up with fellow Georgetown University alumni to watch the debate because he said that what happened in the United States was deeply important to Mexico.

Mr. Lorenzo said the candidates were more concerned about the flow of drugs north from Mexico, but “as long as there’s a market, there will be products.” The same principle existed, Mr. Lorenzo said, with the market for smuggling firearms from the United States to Mexico, a reality that he said worried him and wasn’t addressed by the candidates.

As far as immigration, Mr. Lorenzo wasn’t pleased with Mr. Trump’s comments, particularly about migrants causing violence and death.

“It’s been eight years, three Trump elections, hearing the same message that we all know is a lie,” he said. “We know it’s populism, it’s a lie, infusing fear into the Americans, that the immigrants are coming to take your jobs and rape your families.” He added later, “Crime does not derive from the migrants,” noting that the vast majority go to work.

Even though Mr. Lorenzo said he thought President Biden has tried to blunt soaring migration, he said neither candidate presented a plan to address it, and instead traded barbs.

Sergio García Chavarría, 52, a driver, said the debate was more style than substance, with the political points scored by the candidates’ jabs. He said he, too, wished Mr. Biden or Mr. Trump had explained how he would address immigration if he won.

Mr. García said he thought Mr. Trump “doesn’t understand that the United States was built by immigrants.”

Mr. García, a Mexican who became a U.S. citizen nearly two decades ago through his former wife, an American, said he watched the debate because he will vote in the U.S. presidential election in November and wanted to be informed. (With his dual nationality, he also voted in the Mexican presidential election in June.) So he tagged along with his girlfriend to a Democrats Abroad event at the restaurant.

He said he was leaning toward supporting Mr. Biden but lamented not hearing enough proposals. “He said, ‘This is what I’ve already done.’ But tell me what you’re doing to do.”

Mr. García said he also wished both candidates had discussed how they would curb the drug problem.

Dmitri Mehlhorn, the political adviser to one of the Democrats’ biggest donors, Reid Hoffman, emailed his network just after midnight trying to soothe concerns — without sugarcoating the debate.

“Joe had a horrible night, cementing concerns about his age, his greatest electoral weakness,” he wrote. “Our odds of Trumpocalypse II just materially increased.” But, he added, “we all need to take a deep breath. Reactive or panicky moves rarely succeed.”

The Biden campaign said on Friday that it raised $14 million “on debate day and the morning after” the debate, but it is difficult to directly compare that with Mr. Trump’s numbers. Mr. Trump’s team raised over $8 million solely on Thursday evening, a Trump campaign official said. What also makes both campaigns’ post-debate fund-raising numbers difficult to assess is that the final days of a fund-raising quarter — the current one ends on Sunday — are already among the most lucrative, even without a high-profile news-making event like a general election debate.

The Biden team will have a chance to address concerns at a few events over the next 48 hours, according to copies of invitations seen by The New York Times. On Friday afternoon, Jill Biden, the first lady, is attending a fund-raiser in New York City hosted by several authors. In the evening, the president and Dr. Biden are scheduled to close out Pride Month with a gala for the L.G.B.T. community featuring Pete Buttigieg, the transportation secretary; his husband, Chasten; and celebrities including Alan Cumming, Wilson Cruz and Billy Porter.

On Saturday, more celebrities will toast Mr. Biden at an afternoon reception in East Hampton, N.Y., where the hosts include Michael J. Fox, Sarah Jessica Parker and Matthew Broderick. The Bidens are also expected to travel to Red Bank, N.J., on Saturday for a fund-raiser thrown by Gov. Phil Murphy of New Jersey and his wife, Tammy.

But first, on Friday morning, top Biden campaign officials — including the campaign chair, Jennifer O’Malley Dillon, and the campaign manager, Julie Chavez Rodriguez — filed into the basement of a hotel in downtown Atlanta to huddle with some of the donors and fund-raisers who had traveled to the debate. The location? One floor below a coffee shop called Jittery Joe’s.

Michael Gold and Shane Goldmacher contributed reporting.

“Morning Joe,” on MSNBC, is perhaps television’s strongest redoubt of Biden support. On Friday, host Joe Scarborough told viewers that Biden had failed to deliver a strong performance at the debate, missing “lay-up after lay-up,” and that it was fair now to raise questions about his fitness to continue as the Democratic nominee.

The mood on “Morning Joe” is by turns sad, stunned and defiant. Scarborough expressed deep admiration for Biden, saying his presidency had been “an unqualified success.” But he said the threat of a second Trump term was too great to avoid asking if Biden was up to the task of this race. His co-host, Mika Brzezinski, said that Biden had “a terrible night” — but that she is “not ready to give up” on him.

Donald Trump flew to Virginia after the debate, sleeping at his golf club in Sterling, on the edge of the Washington suburbs. He’ll travel today to Chesapeake, Va., for a rally this afternoon in a state he hopes to flip after years of it voting for Democratic presidential candidates.

In perhaps a sign of what stuck out to Trump from the debate, he posted videos of his golf swing to Truth Social at around 1 a.m. Trump brought up his golf game toward the end of the debate as he responded to a question about his age. He and Biden then traded jabs about their swings and handicaps.

In Europe and Asia, the back-and-forth between the blustering Mr. Trump and the faltering Mr. Biden set analysts fretting — and not just about who might win the election in November.

“That whole thing was an unmitigated disaster,” Simon Canning, a communications manager in Australia, wrote on social media. “A total shambles, from both the candidates and the moderators. America is in very, very deep trouble.”

Sergey Radchenko, a historian at the Johns Hopkins School of Advanced International Studies in Washington, posted, “This election is doing more to discredit American democracy than Vladimir Putin and Xi Jinping could ever hope to,” referring to the presidents of Russia and China, America’s most powerful rivals for global leadership.

“I am worried about the image projected to the outside world,” he continued. “It is not an image of leadership. It is an image of terminal decline.”

Whoever becomes president, the United States faces major global challenges — in Asia, from a rising China and a nuclear North Korea recently bolstered by Mr. Putin; in Europe from Russia’s war against Ukraine; and in the Middle East, where Israel’s war against Hamas threatens to escalate to southern Lebanon and even Iran.

There was little of substance on foreign policy in the noisy debate. Mr. Trump continued to insist without explanation that he could have prevented Mr. Putin from invading Ukraine, or Hamas from invading Israel, and that he could bring a quick end to both conflicts, again without explaining how or at what cost and to whom.

Mr. Biden cited his efforts to bring allies together to aid Ukraine and confront Russia. “I’ve got 50 other nations around the world to support Ukraine, including Japan and South Korea,” he said.

For some, the debate made a Trump presidency, already considered a strong possibility, seem like a probability, said François Heisbourg, a French analyst. “So on all the issues, the debate is a confirmation of European worries, and some of it has already been integrated into people’s thinking.”

On Ukraine, people hear Trump saying he wants to cut back aid to Ukraine, so this will move to the center of the debate,” he said, along with Mr. Trump’s stated fondness for Mr. Putin as a strong leader.

On Israel and Gaza, however, “I’m not sure it will make much of a difference,” Mr. Heisbourg said. “You can’t move the embassy to Jerusalem twice.”

As for the state of American democracy, Mr. Heisbourg sighed. “This is not a new question,” he said. “It’s more a confirmation of what’s happening, including in France.”

Added to existing worries about the unpredictable Mr. Trump, which the debate only confirmed, is fresh anxiety about Mr. Biden’s capacity to govern. One of the harshest assessments came from Radoslaw Sikorski, the Polish foreign minister. In a social media post, he compared Mr. Biden to Marcus Aurelius, the Roman emperor who “screwed up his succession by passing the baton to his feckless son Commodus, whose disastrous rule started Rome’s decline.”

“It’s important to manage one’s ride into the sunset,” Mr. Sikorski added.

The headline in the French daily Le Monde read: “Joe Biden’s shipwreck in the televised debate against Donald Trump.” The president, the newspaper continued, is “a shadow of the Joe Biden who faced Donald Trump in the 2020 election.”

American presidents drive a very big truck with a large number of other countries pulled along behind, said Daniela Schwarzer, an executive board member of the Bertelsmann Foundation, a think tank in Washington, D.C. “And for the next four years, you need a strong American president and a reliable partner to Europe — someone who can stand his ground in a world where there will be more conflict all around,” she said.

In Ukraine, the clamor about the debate reverberated on Friday.

Referring to Mr. Biden, Bogdan Butkevych, a popular radio host, wrote on social media, “His main task was to convince the voters of his energy and readiness to rule.” But, he added, “He wasn’t able to do it. Accordingly, the chance of his replacement by another candidate from the Democrats increases.”

Some took a measure of solace in Mr. Trump’s saying that he did not find it acceptable for the Kremlin to keep occupied lands.

In that vein, The Kyiv Independent, a Ukrainian news outlet, ran a headline about the debate that read, “Trump rejects Putin’s peace terms while Biden unnerves Democrats.”

Elsewhere, countries that have hoped the United States could balance a rising China and deter North Korea’s nuclear ambitions spent the past four years trying to rebuild ties with Washington after Mr. Trump’s first term deeply rattled alliances in the region. The debate on Thursday night immediately resurfaced serious questions about how U.S. politics might affect stability across Asia.

Chan Heng Chee, who served as Singapore’s ambassador to the United States from 1996 to 2012, said that the quality of the debates had deteriorated compared with previous ones. Mr. Biden’s disjointed performance and Mr. Trump’s repeated attacks and factual inaccuracies unsettled those who rely on the United States to act as a trusted global partner.

“Now everyone is watching for visuals,” Ms. Chan said. “Do the candidates look like they are able to do the job, or is age a problem? Facts do not matter now, and civility has totally gone out of the window.”

In Japan and South Korea, analysts detected a shift in the political winds toward Mr. Trump, and it prompted renewed questions about Mr. Biden’s age and ability to project strength.

“It was clearly a Trump win and a nail in the coffin for the Biden campaign,” said Lee Byong-chul, a professor at the Institute for Far Eastern Studies at Kyungnam University in Seoul.

“We must now brace ourselves for a second Trump administration,” he added.

In Japan, a major American ally in Asia, officials have almost always been assiduous about declaring that they are happy working with whomever the United States elects. But Mr. Trump’s comments during the debate about how he does not want to spend money on allies are likely to revive anxieties about how his approach to international relationships is transactional rather than enduring.

“My guess is that the Japanese policymakers are thinking, ‘OK, it’s going to be Trump quite likely, so we have to cement institutional ties as much as possible so he can’t undo them,’” said Koichi Nakano, a political scientist at Sophia University in Tokyo. “That is like tying yourself to a mast that may be sinking very soon, so it’s a false illusion of security.”

India, traditionally averse to sudden change and slow in any foreign policy shift, has worked in recent years to overcome a long history of mistrust to expand military and trade ties with Washington. While Prime Minister Narendra Modi enjoyed a close bond with Mr. Trump during his presidency, the Indian establishment has seen in Mr. Biden a steady hand that understands how alliances work and how geopolitical risk can be contained and mitigated.

Dr. Tara Kartha, a former senior official in the National Security Council of India, said that the state of America’s political leadership was worrying New Delhi. She pointed out that Mr. Trump is unpredictable and could easily shift positions — such as changing his current hard-line approach to China and patching things up if Beijing offers him better terms on a trade deal. That uncertainty makes calculations difficult for India, she added, which shares a border with China and a long rivalry with Beijing.

“We are now hedging with China, we are not going beyond a point precisely because of this,” she said. “Because you are not really sure what’s going to happen to the U.S.”

In China, the presidential debate was a top trending topic on the social media platform Weibo. Official Chinese media outlets largely played it straight, reporting each candidates’ remarks — and their lack of a handshake — without adding much commentary.

But in comments online, some users compared Mr. Trump’s red tie to a Communist red scarf, and some social media commentators jokingly called Trump “nation builder” because of how his leadership could accelerate China’s global rise.

Social media merriment aside, Shen Dingli, a Shanghai-based international relations scholar, said that the debate had only reinforced something the Chinese government had long thought: No matter who the next president is, U.S. policy toward China is only likely to harden.

“I believe Chinese leaders don’t have any illusions,” he said.

What was clear after Thursday’s debate was that few in the region felt optimistic about the electoral options in the United States.

Kasit Piromya, Thailand’s foreign minister from 2008 to 2011 and a former ambassador to the United States, lamented the state of American politics.

“Where are the good ones? Where are the brave ones?” Mr. Kasit said, adding that it was now incumbent on countries in Southeast Asia to have a foreign policy vision of their own.

“Why should I wait for Trump to be bad? I should be able to organize myself and maybe work with other friends,” he said.

Reporting was contributed by Damien Cave, Sui-Lee Wee, Choe Sang-Hun, Vivian Wang, Camille Elemia, Mujib Mashal, Ségolène Le Stradic and Marc Santora.

He indicated that his raspy voice stemmed from a minor ailment. “I have a sore throat,” he said. His aides said he had been fighting a cold.

Mr. Biden then headed to an Air Force base for a late-night flight to Raleigh, N.C., where he plans to hold a rally on Friday.

She conceded that he did not perform expertly at the start of the debate.

“It was a slow start, that’s obvious to everyone,” Ms. Harris said. “I’m not going to debate that.”

At a virtual debate watch party with supporters before her CNN appearance, Ms. Harris appeared to read from prepared remarks to assure supporters.

“He got into a groove where it counted,” Ms. Harris said in her remarks. “Our president showed that he will win the election.”

On CNN, she argued that the election must be decided “on substance,” not on debate style. And she sought to highlight the false claims made by former President Donald J. Trump throughout the debate and raise alarm about how he might restrict abortion access if he returns to office.

In the MSNBC interview that followed, she repeatedly described Mr. Biden as “clear” in his messaging and said that, during the debate in particular, his pitch to enshrine abortion access in a second term was firm.

Ms. Harris conceded again that Mr. Biden had a slow start, but added that “I thought it was a strong finish.”

Ms. Harris is expected to address supporters at a rally in Las Vegas on Friday.

He tried to accuse Mr. Biden of corruption, dubbing the president as a “Manchurian candidate” who was “paid by China,” a nod to frequent accusations of undue influence for which there is no evidence.

He directly blamed Mr. Biden for a wave of immigrants “coming in and killing our citizens at a level we’ve never” seen, a hyperbolic claim that is not backed up by available statistics.

And in a wild misrepresentation of facts, Mr. Trump claimed falsely that Mr. Biden “encouraged” Russia to attack Ukraine, even though Mr. Biden has consistently tried to rally support for Ukraine and his administration took active steps to warn President Vladimir V. Putin of Russia not to invade.

Mr. Trump’s remarks during the debate were not substantially different from the way he typically inveighs against Mr. Biden during his rallies, where he depicts the president as a leader who is somehow both bumbling and corrupt as he steers the country to ruin.

But the barrage of attacks during the debate was particularly striking given that Mr. Biden was standing mere feet away from him, unable to interrupt or effectively challenge Mr. Trump because of debate rules that kept his microphone muted.

And as the debate’s moderators, the CNN anchors Jake Tapper and Dana Bash, focused on keeping the peace, they did not even try to fact-check Mr. Trump’s assertions, allowing them to stand unchallenged.

Mr. Biden got in a few licks, including some of the debate’s more memorable moments. He said Mr. Trump had the “morals of an alley cat” and accused him of having sex with a porn star while his wife was pregnant.

But by and large, Mr. Biden was on the defensive from the get-go in the face of a steady stream of insults, false characterizations and attacks from Mr. Trump.

Seizing on Mr. Biden’s halting speech early in the debate, Mr. Trump pounced at one moment when Mr. Biden trailed off, saying: “I really don’t know what he said at the end of that sentence. I don’t think he knows what he said, either.”

But Mr. Trump’s most forceful attacks surrounded immigration, an issue that animated his successful 2016 campaign and that he has tried to put at the center of his bid to return to the White House.

The former president invoked the idea of “Biden migrant crime,” claiming that Mr. Biden’s lax border policy had allowed terrorists and criminals to cross the border illegally.

Mr. Trump accused his successor of “ridiculous, insane and very stupid policies” that fostered a crime wave, pointing to high-profile killings that involved immigrants. He vaguely accused Mr. Biden of killing “so many at our border” by not curbing the surge of migrants, an assertion that he did not back up with statistics.

Experts have said that those heavily publicized cases do not represent a broader trend. Studies have concluded that immigration does not push up crime rates.

Mr. Trump also went directly after Mr. Biden’s profile on the world stage. He argued that Mr. Putin was “laughing at” the president’s leadership and at his failure to secure the release of Evan Gershkovich, the Wall Street Journal reporter detained in Russia on an espionage charge that American officials vehemently deny.

”Our whole country is exploding because they don’t respect you,” Mr. Trump told Mr. Biden.

He extended those criticisms to the military, arguing that “our veterans and our soldiers can’t stand” the president. (Mr. Trump, while in office, reportedly denigrated senior American military officials.)

In repeating his frequent assertions that Mr. Biden is corrupt, Mr. Trump revived his accusations that Mr. Biden improperly received payments from a Chinese energy company associated with his son Hunter and his brother James. There is no evidence that any portion of those payments — which started after Mr. Biden left the vice presidency — went to the president.

But Mr. Trump also directly attacked Hunter Biden, who was found guilty this month on three felony counts related to his buying a gun while grappling with drug addiction. He called Hunter “a convicted felon at a very high level.”

Mr. Trump was convicted last month on 34 felony charges in Manhattan related to hush-money payments to a porn star.

Mr. Biden, the 46th president, entered Thursday’s debate with the 45th, Donald J. Trump, needing to calm concerns about his own age and mental acuity. Instead, Mr. Biden’s incoherent performance inflamed those fears, raising questions from the start about whether he would be able to carry on as the Democratic nominee.

Any talk of Mr. Biden’s relying on performance-enhancing drugs to survive the debate seemed quaint by the time he opened his mouth on the debate stage and suffered what seemed like a prime-time meltdown.

Even Mr. Trump looked almost taken aback as his adversary stumbled and struggled to get his words out. “I really don’t know what he said at the end of that sentence,” Mr. Trump said after Mr. Biden answered a question about border security not 10 minutes into their debate. “I don’t think he knew, either.”

It was all the more startling considering Mr. Biden had cleared his calendar and holed up at Camp David for days to prepare. (White House officials said that Mr. Biden had a cold.)

Mr. Trump’s message was often factually incorrect, but it was one communicated clearly, fiercely and impatiently. Mr. Biden, in contrast, stood slack-jawed, his eyes darting back and forth, while his opponent spoke.

Early in the debate, Mr. Biden briefly seemed to go blank, stumbling in a syntax-free way to the end of a long point he was trying to make about health care. He finally seemed to give up, saying, “Look. If — we finally beat Medicare.”

The strangeness of the moment did not prevent Mr. Trump from pouncing: “Well, he’s right. He did beat Medicare — he beat it to death.”

Mr. Biden tried some of the lines he leaned on in earlier debates — “I’ve never heard so much malarkey in my life,” he said at one point — but it underscored only how different he sounded now, even compared with just four years ago.

Gone was the confident ear-to-ear grin in the split screen.

At times, Mr. Biden seemed to want to go for the jugular, as when he invoked the hush-money payments Mr. Trump made to a porn star that resulted in 34 felony convictions. But the attack came out garbled, ending rather lamely as Mr. Biden described his opponent as having “the morals of an alley cat.”

Mr. Biden repeatedly interrupted himself and trailed off mid-answer. While discussing abortion, arguably his strongest issue, he interjected a mention of immigration and crime, his weakest. “A young woman who just was murdered, and he went to the funeral. The idea that she was murdered by a — by an immigrant coming into — they talk about that,” he said.

Later, he seemed to come up with a new way to define the three trimesters of pregnancy.

“I supported Roe v. Wade, which had three trimesters,” he said. “First time is between a woman and a doctor. Second time is between the doctor and an extreme situation. A third time is between the doctor. I mean, it would be between the woman and the state.”

At times, such as in an exchange about Mr. Trump’s reported comments about veterans, Mr. Biden seemed enraged and yet not quite able to express his anger. At other points, as when he talked about climate change and historically Black colleges and universities, he seemed almost out of breath.

Talking about the war in Ukraine, Mr. Biden seemed to confuse Mr. Trump with Mr. Putin, saying: “If you take a look at what Trump did in Ukraine, he, this guy told Ukraine, told Trump, do whatever you want, do whatever you want, and that’s exactly what Trump did. Putin encouraged him, do whatever you want. And he went in.”

Toward the end of the debate, Mr. Biden appeared to perk up while talking about affordable child care. And he landed a blow when he called Mr. Trump a “whiner” who “snapped” when he lost the election in 2020. But by then, the hour was late.