US President Joe Biden delivered an uneven performance while his Republican rival Donald Trump battered him with a series of often false attacks at their debate on Thursday, as the two oldest presidential candidates ever exchanged personal insults ahead of the November election.

The two men traded barbs on abortion, immigration, the wars in Ukraine and Gaza, their handling of the economy and even their golf games as they each sought to shake up what opinion polls show has been a virtually tied race for months.

A hoarse-sounding Biden stumbled over his words on several occasions during the debate's first half-hour. But he found his footing at the halfway mark when he attacked Trump over his conviction for covering up hush money payments to porn star Stormy Daniels, calling him a "felon."

In response, Trump brought up the recent conviction of Biden's son, Hunter, for lying about his drug use to buy a gun.

Moments later, Biden noted that almost all of Trump's former cabinet members, including former Vice President Mike Pence, have not endorsed his campaign. "They know him well, they served with him," he said. "Why are they not endorsing him?"

Trump, meanwhile, unleashed a barrage of criticisms, many of which were well-worn falsehoods he has long repeated, including claims that migrants have carried out a crime wave, that Democrats support infanticide and that he actually won the 2020 election.

But Biden's 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 the panic had set in,” said David Axelrod, a longtime advisor to former President Barack Obama on CNN, immediately after the debate about Biden's performance.

“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.”

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

One top Biden donor, who did not want to be identified while criticising the president, called his performance "disqualifying" and said he expected a fresh round of calls for him to step aside ahead of the party's national convention in August.

Vice President Kamala Harris, appearing on CNN after the debate, acknowledged what she called Biden's "slow start" but argued that voters should judge him and Trump based on their years in office.

Asked about his performance in the debate, Biden told reporters afterwards that “I think we did well,” but said he has a sore throat. Pressed about Democratic concerns with his showing that he should consider stepping aside, Biden said, "No, it's hard to debate a liar.”

Biden, 81, and Trump, 78, were both under pressure to display their fitness for office.

Biden has been dogged by questions about his age and sharpness, while Trump's incendiary rhetoric and sprawling legal woes remain a vulnerability.

Many television viewers took to social media to complain, describing the debate as a "disaster," a "train wreck," and a "waste of time," expressing concerns about America's future.

Multiple polls show that the majority of American voters are weary of another Biden-Trump matchup in the 2024 presidential election, with the proportion of voters holding negative views of both candidates reaching the highest level in 30 years.

The second presidential debate for the 2024 U.S. election, to be hosted by ABC, is scheduled to take place on September 10. (Agencies)

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Last updated: 2024-06-28 HKT 15:23