Biden noticeably stumbled at times as he delivered his retorts with a thin voice, mumbled words and occasional look of confusion — a performance that drew immediate worries among Democrats concerned about his age and perceived fragility among many voters.

Trump, who attempted to deliver a less voluble and defiant performance than past debates, based many of his answers around falsehoods that went unchallenged by the debate moderators.

In the sharpest exchanges of the night, Biden accused Trump of having “the morals of an alley cat” for allegedly having “sex with a porn star,” a reference to allegations Trump still denies that were a part of his recent criminal trial in New York.

Biden called Trump a “convicted felon” and “such a whiner” and also denounced Trump for his refusal to condemn those who committed violence in the deadly Jan. 6, 2021, attack on the U.S. Capitol.

“The idea that you have the right to seek retribution against any American is just wrong. No president has ever spoken like that before,” he said, before predicting that Trump is unlikely to accept the results of the coming election if he loses.

Trump, in turn, called Biden’s actions “absolutely criminal,” and falsely alleged that Biden “gets paid by China” and is a “Manchurian candidate,” assertions that are not backed up by any evidence. Trump, unprompted, also brought up the recent criminal conviction of Biden’s son, Hunter.

“He’s not equipped to be president. You know it and I know it. It’s ridiculous,” Trump said at one point. “We have a debate. We’re trying to justify his presidency. His presidency is, without question, the worst president, the worst presidency in the history of our country. We shouldn’t be having a debate about it. There’s nothing to debate.”

Trump told Biden that he should be ashamed about Justice Department prosecutions connected to the Jan. 6 attack. Later he said of Biden, “Everything he does is a lie. It’s misinformation and disinformation.”

Biden, 81, came into the debate aiming to cast Trump as a more unhinged and extreme version of the politician that voters elected in 2016, while Trump, 78, looked to capitalize on voter perceptions that Biden has been a less successful president and now faces diminishing faculties because of age.

Biden did little to put to rest voter perceptions that his age is a major issue in this election, largely avoiding a question about voter worries that he would be 86 years old at the end of his second term.

Democrats expressed concern after the debate that Biden had done little to settle concerns about his ability to serve another term as president, and even some campaign officials acknowledged that Biden may have damaged his candidacy. “It was a really disappointing debate performance from Joe Biden. I don’t think there is any other way to slice it,” said Kate Bedingfield, Biden’s former communications adviser, on CNN after the debate ended.

Twelve minutes in, Biden appeared to lose his train of thought while answering a question about the deficit. He mixed up “trillionaires” for “billionaires” and “millions” for “billions” while starting to explain that taxing the ultra-wealthy would make “every single solitary person eligible for what I’ve been able to do with the covid.”

He stopped to correct himself. “Excuse me, with the, dealing with everything we have to do with —”

He looked down and trailed off, then finished with “look, if — we finally beat Medicare.”

As Biden’s time ran out, CNN’s moderator Jake Tapper stepped in and gave the floor to Trump, who seized on Biden’s words.

“Well, he’s right, he did beat Medicare,” Trump said.

Ten minutes later, after Biden gave another mumbling answer, 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.”

Trump later challenged Biden to take a cognitive test and argued that he did not think Biden had the ability to hit a golf ball 50 yards.

Biden responded by saying he had recently been a six handicap and would play Trump in golf if Trump agreed to carry his own bag. “Just take a look at what he says he is and look at what he is,” Biden said.

“I’ve seen your swing. I know your swing,” Trump responded, calling the Biden claim of a six handicap false.

Biden’s performance improved as the debate continued. His voice cleared and he appeared more comfortable on stage. Trump, for his part, made a point of honoring the strict rules that had been imposed by Biden’s campaign, promptly yielding the floor and maintaining his composure while Biden was speaking. Each candidate’s microphone was turned off when they were not speaking in an effort to limit cross-talk.

The exchange over age and fitness was one of the only times the two men interrupted one another during the 90-minute debate.

“Let’s not act like children,” Trump said to Biden.

“You are a child,” Biden responded.

The candidates were not able to bring notes or speak with their advisers during the two commercial breaks during the roughly 90-minute meeting at CNN’s television studios. The moderators, anchors Tapper and Dana Bash, did not fact-check the candidates during the debate, which moved along swiftly from topic to topic as the candidates spoke in one- or two-minute chunks.

While speaking about abortion restrictions, Biden referenced victims of rape and incest who are forced to carry pregnancies to term under some Republican state legislation and proposals. Trump then accused Biden of letting criminals come into the country, part of a pattern of pivoting to immigration throughout the debate.

“He’s the one that killed people with a bad border, including hundreds of thousands of people dying and also killing our citizens when they come in,” Trump said. “We are living right now in a rat’s nest. … We’re literally an uncivilized country now. He doesn’t want it to be. He just doesn’t know.”

Trump alleged, as he often does in campaign speeches, that foreign countries are emptying jails and mental hospitals to send people to the United States — a claim that he has not provided any evidence to support. There is no research that establishes a connection between immigration and crime.

“The idea we’re emptying our prisons, we’re welcoming these people, it’s simply not true. There’s no data to support what you said,” Biden said. “Once again, he’s exaggerating. He’s lying.”

Later, Biden faulted Trump for blocking a bipartisan compromise for boosting border security and for family separations at the border during his presidency.

“He’s the worst president in the history of our country. He destroyed our country,” Trump said at another point. “He wants open borders. He wants either to be destroyed or he wants to pick up those people as voters.”

Biden was ready for the last attack.

“We are the most admired country in the world. We are the United States of America,” Biden said. “There’s nothing beyond our capacity. We are the finest military in the history of the world, the finest in the history of the world. No one thinks we’re weak. No one wants to screw around with us.”

On foreign policy, the two men presented sharply contrasting visions on the wars in Ukraine and Gaza, conflicts in which U.S. weaponry has played an outsize and increasingly controversial role.

Highlighting the proliferations of conflicts in recent years, Trump looked at Biden and said, “The whole world is blowing up under him.”

In response, Biden quipped: “I’ve never heard so much malarkey in my life.” He suggested that Trump would walk away from NATO and allow Russian president Vladimir Putin to have his way with Ukraine.

Trump took Biden to task over the war in Ukraine, saying that the billions of dollars in taxpayer funding going to that conflict were unnecessary.

“That’s a lot of money,” Trump said. “The money that we’re spending on this war — and we shouldn’t be spending. It should’ve never happened.”

The Biden administration has secured more than $100 billion in taxpayer funding to support Ukraine in the wake of Russia’s 2022 invasion, but the war has lost some backing among Americans as the costs have piled up and the fighting has dragged along.

On Israel, Biden stood by his administration’s handling of the war, saying “we saved Israel” and asserting that the militant group Hamas “should be eliminated.”

Trump responded by attacking Biden over the conflict, suggesting that Biden should be more supportive of Israel.

“He’s become like a Palestinian, but they don’t like him because he’s a very bad Palestinian,” Trump said of Biden. “He’s a weak one.”

Trump responded to a direct question about his role in encouraging his supporters to attack the U.S. Capitol by changing the subject to the state of the economy on that day.

Tapper pressed him by repeating his question, and Trump falsely shifted blame for the attack to then-House Speaker Nancy Pelosi (D-Calif.) and D.C. Mayor Muriel Bowser, then attacked the bipartisan House committee that investigated the assault.

Biden pushed back: “He sat there for three hours, three hours, watching, being begged by his vice president and a number of his colleagues and Republicans as well, to do something to call for a stop to end it.” He then criticized Trump for pledging to pardon convicted rioters, saying, “He wants to let them all out, and now he says if he loses it again, such a whiner that he is, that it could be a bloodbath.”

Biden finished by pointedly asking Trump, as he did in their first debate in 2020, to denounce violent extremist groups such as the Proud Boys and others who attacked the Capitol. Trump did not answer, and Tapper moved on to the next question.

Trump also avoided repeated questions about whether he would accept the results of the 2024 election once all legal challenges were exhausted. After he was asked the third time, he responded, “If it is a fair and legal and good election, absolutely.”

Trump entered the debate in a substantially better position than during their 2020 matchup, with polling leads outside the normal-sized polling error in all battleground states except for Wisconsin, Michigan and Pennsylvania, where the race is roughly tied. Biden’s favorability rating in the Gallup Poll has fallen four points to 37 percent between December and June, while Trump’s favorability rose four points to 46 percent, despite his felony convictions in a New York courtroom.

Much of Biden’s lost standing since 2020 comes from cracks in his own coalition, with historically large shares of Democrats continuing to voice displeasure that he is their nominee. Biden expressed confidence that the numbers would shift in his favor in the coming months.

“Let’s see where your numbers are when this election is over,” Biden said to Trump.

Breaking with tradition of three presidential debates, Biden has said he will participate in only one more event with Trump, an ABC News event on Sept. 10, with similar rules and format. Trump has called for additional meetings to be scheduled.

The Biden campaign has also proposed an additional vice presidential debate after the Republican nominating convention in July.

Scherer reported from Washington. Tyler Pager contributed to this report.