US President Joe Biden delivered a shaky, halting 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.

Biden's allies tried to put a brave face on the evening, and two White House officials said Biden had a cold.

But the president's poor performance rattled his fellow Democrats and will likely deepen voter concerns that the 81-year-old is too old to serve another four-year term.

One top Biden donor, who did not want to be identified while criticizing 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.

"I'm not going to spend all night with you talking about the last 90 minutes when I've been watching the last three-and-a-half years of performance," she told CNN host Anderson Cooper.

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.

Biden 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.

"Obviously, the biggest factor is that Biden still seemed old and raspy and less coherent than when he ran last time," said Matt Grossmann, a political science professor at Michigan State University. "I don't think Trump really did anything to help himself beyond his existing supporters, but I think it's eclipsed by people's impressions of Biden on his biggest vulnerability."

Asked about the Jan. 6, 2021, attack on the U.S. Capitol by a mob of Trump supporters, the former president refused to accept any responsibility and claimed that many of those arrested were innocent.

"This guy has no sense of American democracy," Biden scoffed in response.

Biden also blamed Trump for enabling the elimination of a nationwide right to abortion by appointing conservatives to the U.S. Supreme Court, an issue that has bedeviled Republicans since 2022.

Trump countered that Biden would not support any limits on abortions and said that returning the issue to the states was the right course of action.

Trump said Biden had failed to secure the southern U.S. border, ushering in scores of criminals.

"I call it Biden migrant crime," he said.

Biden replied, "Once again, he's exaggerating, he's lying."

Studies show immigrants do not commit crimes at a higher rate than native-born Americans.

NAME-CALLING

The televised 90-minute clash on CNN took place far earlier than any modern presidential debate, more than four months before the Nov. 5 Election Day.

The two candidates appeared with no live audience, and their microphones automatically cut off when it was not their turn to speak - both atypical rules imposed to avoid the chaos that derailed their first debate in 2020, when Trump interrupted Biden repeatedly.

The two men - who have made little secret of their mutual dislike - did not shake hands or acknowledge each other before or after the debate.

But there were plenty more moments in which their bad blood was evident. Each called the other the worst president in history; Biden referred to Trump as a "loser" and a "whiner," while Trump called Biden a "disaster."

At one point, the rivals bickered over their golf games, with Trump bragging about hitting the ball farther than Biden and Biden retorting that Trump would struggle to carry his own bag.

The first question focused on the economy, as polls show Americans are dissatisfied with Biden's performance despite wage growth and low unemployment.

Biden acknowledged that inflation had driven prices substantially higher than at the start of his term but said he deserves credit for putting "things back together again" following the coronavirus pandemic.

Trump asserted that he had overseen "the greatest economy in the history of our country" before the pandemic struck and said he took action to prevent the economic freefall from deepening even further.

The debate took place at a time of profound polarization and deep-seated anxiety among voters about the state of American politics. Two-thirds of voters said in a May Reuters/Ipsos poll that they were concerned violence could follow the election, nearly four years after a mob of Trump supporters stormed the U.S. Capitol.

Trump took the stage as a felon who still faces a trio of criminal cases, including for his efforts to overturn the 2020 election. The former president, who persists in falsely claiming his defeat was the result of fraud, has suggested he will punish his political enemies if returned to power, but he will need to convince undecided voters that he does not pose a mortal threat to democracy, as Biden asserts.

Biden's challenge was to deliver a forceful performance after months of Republican assertions that his faculties have dulled with age.

While national polls show a tied race, Biden has trailed Trump in polls of most battleground states that traditionally decide presidential elections. Just this month he lost his financial edge over Trump, whose fundraising surged after he was criminally convicted of trying to cover up hush money payments to porn star Stormy Daniels.

Neither Biden nor Trump is popular and many Americans remain deeply ambivalent about their choices. About a fifth of voters say they have not picked a candidate, are leaning toward a third-party candidate or may sit the election out, the latest Reuters/Ipsos poll showed.

The second and final debate in this year's campaign is scheduled for September.