A fired-up Joe Biden came out swinging Friday as he tried to make up for a disastrous debate performance against Donald Trump, insisting he was the right man to win November's US presidential election.

Biden's appearance at a campaign rally in the battleground state of North Carolina came amid rumblings in his alarmed Democratic Party about replacing the 81-year-old as their nominee -- and shortly before the nation's most influential newspaper urged him to step aside.

"I don't walk as easy as I used to. I don't speak as smoothly as I used to. I don't debate as well as I used to," Biden admitted to supporters in unusually confessional remarks.

"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."

Biden's team was in damage-control mode after Thursday's debate when he often 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 expose Trump as a habitual liar.

But the president failed to counter his bombastic rival, who offered up a largely unchallenged reel of false or misleading statements about everything from the economy to immigration.

On Friday, Biden delivered the lines Democrats wished they had heard in the televised debate.

"Did you see Trump last night? My guess is he set -- and I mean this sincerely -- a new record for the most lies told in a single debate," Biden said.

"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."

Trump also returned to the campaign trail Friday, speaking at a rally in Virginia and launching his familiar attacks on Biden in a rambling speech.

"It's not his age, it's his competence," Trump said.

"The question every voter should be asking themselves today is not whether Joe Biden can survive a 90-minute debate performance, but whether America can survive four more years of crooked Joe Biden."

A new Democrat?

Trump addressed the chances of Biden being replaced by another candidate, saying "I don't really believe that because he does better in polls than any of the (other) Democrats."

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.

"I will never turn my back on President Biden," California Governor Gavin Newsom, who has figured prominently on lists of possible replacement candidates, said immediately after the debate.

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 next month.

Biden overwhelmingly won the primary votes, and the party's 3,900 delegates heading to the convention in Chicago are beholden to him.

If he exits, the delegates would have to find a replacement.

"Bad debate nights happen," Biden's former boss, Barack Obama, wrote on X.

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."

A logical -- but not automatic -- candidate to take Biden's place would be his vice president, Kamala Harris, who also loyally defended his debate performance.

The show of Democratic loyalty and Biden's defiance in North Carolina were not enough for The New York Times, however.

The daily newspaper slammed Biden's campaign as a "reckless gamble" in the face of the threat posed by Trump, with its editorial board -- which is separate from the newsroom -- calling for the president to stand aside. The "greatest public service Mr. Biden can now perform is to announce that he will not continue to run for re-election," it said.

Many election bettors, too, abandoned Biden, preferring to bet on Trump or other Democratic leaders. Before the debate, bettors on the platform Smarkets were giving Biden a 35 percent chance of winning in November, but on Friday that figure dropped to below 20 percent.

A second debate is scheduled for September 10. (AFP)