A raspy and sometimes halting President Joe Biden tried repeatedly to confront Donald Trump in their first debate ahead of the November election, as his Republican rival countered Biden's criticism by leaning into falsehoods about the economy, illegal immigration and his role in the January 6, 2021, Capitol insurrection.

Biden's uneven performance, particularly early in the debate, crystallised the concerns of many Americans that, at 81, he is too old to serve as president.

It sparked a fresh round of calls for the Democrat to consider stepping aside as the party fears a return of Trump to the White House.

Biden repeatedly tore into Trump in an apparent effort to provoke him, bringing up everything from the former president's recent felony conviction to his alleged insult of World War I veterans to his weight and golf game.

The 78-year-old Trump declined to clearly state he would accept the results of the November election, four years after he promoted conspiracy theories about his loss that culminated in the January 6 insurrection, and repeatedly misstated the record from his time in his office.

But Biden's halting delivery from the beginning of the debate drew the most attention afterward. Trump's allies immediately declared victory while prominent Democrats publicly questioned whether Biden could move forward.

“I think there was a sense of shock, actually, of how he came out at the beginning of this debate, how his voice sounded. He seemed a little disoriented. He did get stronger as the debate went on but by that time, I think the panic had set in,” said David Axelrod, a longtime advisor to former President Barack Obama, said on CNN immediately after.

“And I think you're going to hear discussions that, I don't know will lead to anything, but there are going to be discussions about whether he should continue.”

Said Van Jones, another Democratic strategist, on CNN: “He did not do well at all.”

Vice President Kamala Harris, speaking on the network afterward, sought to defend the president's performance while acknowledging the criticism.

“There was a slow start, but there was a strong finish,” she said.

Biden began the night with a raspy voice and a halting delivery as he tried to defend his economic record and criticise Trump. A person familiar with the matter said Biden was suffering from a cold during the debate, adding that he tested negative for COVID-19.

Biden appeared to lose his train of thought while giving one answer, drifting from an answer on tax policy to health policy, at one point using the word “COVID,” and then saying, “excuse me, with, dealing with,” and he trailed off again.

“Look, we finally beat Medicare,” Biden said, as his time ran out on his answer.

He also fumbled on abortion rights, one of the most important issues for Democrats in this year's election. He was unable to explain Roe v. Wade, the landmark Supreme Court ruling that legalised abortion.

When asked if he supports some restrictions on abortion, Biden said he “supports Roe v. Wade, which had three trimesters. The first time is between a woman and a doctor. Second time is between a doctor and an extreme situation. A third time is between the doctor, I mean, between the women and the state.”

He added that he thought doctors, not politicians, should make decisions about “women's health.”

Biden began to give clearer answers as the debate progressed, still with a rasp, and attacked Trump's record on issues like fighting climate change.

“The only existential threat to humanity is climate change, and he didn't do a damn thing about it,” he said.

Addressing supporters briefly at a watch party near the debate venue, Biden didn't address his performance directly, but said, “let's keep going,” and indicated he has no plans to leave the race. “See you at the next one,” he added.

The current president and his predecessor hadn't spoken since their last debate weeks before the 2020 presidential election.

Trump skipped Biden's inauguration after leading an unprecedented and unsuccessful effort to overturn his loss that culminated in the Capitol riot by his supporters.

Trump equivocated on whether he would accept the results of the November election, saying he would accept them if the vote was “fair” and “legal,” repeating his baseless claims of widespread fraud and misconduct in his 2020 loss to Biden that he still denies.

Pressed on his actions on January 6, 2021, Trump was unapologetic.

“On January 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,” Trump said.

After he was prompted by a moderator to answer whether he violated his oath of office that day by rallying his supporters seeking to block the certification of Biden's Electoral College victory and not acting for hours to call them off as they raided the Capitol, Trump sought to blame then-House Speaker Nancy Pelosi.

Biden said Trump encouraged the supporters to go to the Capitol and sat in the White House without taking action as they fought with police officers.

“He didn't do a damn thing and these people should be in jail,” Biden said. “They should be the ones that are being held accountable. And he wants to let them all out. And now he says that if he loses again, such a whiner that he is, that this could be a bloodbath'?”

Trump then defended the people convicted and imprisoned for their role in the insurrection, saying to Biden, “What they've done to some people that are so innocent, you ought to be ashamed of yourself.”

Trump and Biden entered the night facing stiff headwinds, including a public weary of the tumult of partisan politics and broadly dissatisfied with both, according to polling.

But the debate was highlighting how they have sharply different visions on virtually every core issue — abortion, the economy and foreign policy — and deep hostility toward each other.

Their personal animus quickly came to the surface. Biden got personal in evoking his son, Beau, who served in Iraq before dying of brain cancer. The president criticised Trump for reportedly calling Americans killed in battle “suckers and losers.” Biden told Trump, “My son was not a loser, was not a sucker. You're the sucker. You're the loser.”

Trump said he never said that — a line attributed to Trump by his former chief of staff — and slammed Biden for the chaotic withdrawal of US forces from Afghanistan, calling it “the most embarrassing day in the history of our country's life.”

Trump himself agreed to the withdrawal with the Taliban a year before he left office.

Biden directly mentioned Trump's conviction in the New York hush money trial, saying, “You have the morals of an alley cat," and referencing the allegations in the case that Trump had sex with a porn actress.

“I did not have sex with a porn star," replied Trump, who chose not to testify at his trial.

Pressed to defend rising inflation since he took office, Biden pinned it on the situation he inherited from Trump amid the COVID-19 pandemic.

Biden said that when Trump left office, “things were in chaos.” Trump disagreed, declaring that during his term in the White House, “Everything was rocking good.”

By the time Trump left office, America was still grappling with the pandemic and during his final hours in office, the death toll eclipsed 400,000. The virus continued to ravage the country and the death toll hit 1 million over a year later.

Trump was asked what he would do to make childcare more affordable. He used his answer to instead boast about how many people he fired during his term, including former FBI Director James Comey and criticised Biden for not firing people from his administration.

Prior to the debate, about 6 in 10 US adults (59 per cent) say they were “very concerned” that Biden is too old to be president, according to Gallup data collected in June. Only 18 per cent had the same level of concern about Trump. The poll found Biden's age was also causing alarm among some Democrats: 31 per cent said they were very concerned.

But Trump allies would enter the post-debate spin room triumphant. Trump senior adviser Chris LaCivita called it “the most lopsided win in debate history” and mocked the Biden campaign for saying the president had a cold.

California Gov. Gavin Newsom, a high-profile Democratic supporter of Biden, was pressed on whether he would consider stepping in for Biden. He dismissed the questions, saying, “I will never turn my back on him.”

He said he knows Biden and what he's capable of and said, “I have no trepidation.”