“But I know how to tell the truth. I know how to do this job,” he said to huge cheers, vowing “when you get knocked down, you get back up”.

02:48

US presidential debate: Biden and Trump spar over economy, war in Ukraine

US presidential debate: Biden and Trump spar over economy, war in Ukraine

Biden’s team was in damage-control mode after Thursday’s debate when he repeatedly hesitated, tripped over words and lost his train of thought – exacerbating fears about his ability to serve another term.

He had hoped to allay qualms about his advanced age and to paint Trump as an existential threat to the future of the United States.

But the president struggled to counter his bombastic rival, who delivered an unchallenged reel of false or misleading statements about everything from the economy to immigration.

On Friday, Biden delivered the lines that his supporters wished he could have produced in the televised debate as the election looms a little over four months away.

“Did you see Trump last night? My guess is he set a new record for the most lies told in the single debate,” Biden said.

“I would not be running again if I didn’t believe with all my heart and soul I can do this job because, quite frankly, the stakes are too high.

“Donald Trump is a genuine threat to this nation. He’s a threat to our freedom. He’s a threat to our democracy. He’s literally a threat for everything America stands for.”

America’s most influential newspaper, The New York Times, called in an editorial on Friday for Biden to step aside and allow another Democrat to challenge Trump.

Describing Biden as “the shadow of a great public servant”, the newspaper’s editorial board – which is separate from its newsroom – said Thursday’s debate proved the president had “failed his own test”.

His determination to run again is a “reckless gamble,” it said, adding: “The greatest public service Mr Biden can now perform is to announce that he will not continue to run for re-election.”

As the shock over Biden’s poor showing registered with Democrats, there was growing talk of how the process might work to find a new candidate before the party’s convention in August.

So far, no senior Democratic figure has publicly called on Biden to withdraw, with most toeing a party line about sticking with the existing ticket.

“Bad debate nights happen,” Biden’s former boss, Barack Obama, wrote on X, formerly Twitter.

But the election is “still a choice between someone who has fought for ordinary folks his entire life and someone who only cares about himself. Between someone who tells the truth; who knows right from wrong … and someone who lies through his teeth for his own benefit.

“Last night didn’t change that, and it’s why so much is at stake in November.”

Analysts say forcing a change in the ticket would be politically fraught, and Biden would have to decide himself to withdraw to make way for another nominee before the party convention.

A strong – but not automatic – candidate to take Biden’s place would be his vice-president, Kamala Harris, who loyally defended his performance on Thursday while acknowledging he had made a “slow start”.

As both candidates returned to the campaign trail, Trump told supporters at an afternoon rally in Chesapeake, Virginia, on Friday that he had had a “big victory against a man looking to destroy our country”.

“Joe Biden’s problem is not his age,” Trump said. “It’s his competence.”

Trump advisers said they thought the debate would bolster their chances in Democratic-leaning states like Virginia, which has not backed a Republican presidential candidate since 2004.

His allies sought to project calm assurance as the Democrats scrambled.

US House Speaker Mike Johnson, a senior Republican figure, said it was clear that Biden was not “up to the job.”

“Donald Trump is the only man on that stage that’s qualified and capable of serving as the next president. The election cannot get here soon enough.”

Additional reporting by Reuters