WASHINGTON   -   US President Joe Biden has hit back at criticism over his age, telling supporters in a fiery speech that he will win re-election in November after a poor debate performance fuelled concern about his candidacy.

“I know I’m not a young man, to state the obvious,” he told a rally in the battleground state of North Carolina on Friday, one day after he struggled in the televised showdown with his Republican rival Donald Trump. “I don’t walk as easy as I used to... I don’t debate as well as I used to,” he acknowledged. “But I know what I do know, I know how to tell the truth [and] I know how to do this job.”

Mr Biden, 81, said he believed with his “heart and soul” that he could serve another term, as the cheering crowd in Raleigh chanted “four more years”. While questions over Mr Biden’s age are not new, his shaky performance on the debate stage - which was marked by verbal blanks, a hoarse voice and some difficult-to-follow answers - triggered panic among some Democrats who raised fresh questions about his candidacy.

Mr Biden’s campaign has remained emphatic that the country’s oldest presidential candidate is still fit to serve another term. Campaign officials said he would not step aside for another nominee, despite the poor performance.

“Absolutely not,” Mia Ehrenberg, a Biden campaign spokesperson, said in response to questions about whether Mr Biden would vacate his candidacy. And while several campaign surrogates - including Vice President Kamala Harris - admitted in interviews that the president stumbled during the debate, they reiterated their commitment to the elder statesman and stressed that his answers were more substantive than Mr Trump’s.

“The president might have lost the debate on style, but he won it on facts, he won it on decency, and he won it on the ideas people think are important in this country,” campaign co-chair Mitch Landrieu said on CNN the morning after the debate.

In the days since, many senior Democrats and Biden allies have made additional efforts to defend the former president’s performance and calm liberal jitters.

Former President Barack Obama, who remains one of the party’s most popular figures, tweeted that “bad debate nights happen”.

“This election is still a choice between someone who fought for ordinary folks his entire life and someone who only cares about himself,” Mr Obama wrote, adding that Mr Trump is “someone who lies through his teeth for his own benefit”.

But Democratic fears about Mr Biden are far from quashed.

Party officials, political operatives, and people close to the president who spoke to the BBC’s Katty Kay painted a picture of an anxious party concerned about the strength of their candidate. Nancy Pelosi, the former Democratic House speaker, said that “from a performance standpoint it wasn’t great”. Other Democrats, such as Biden’s former communications director Kate Bedingfield, called it “a really disappointing debate performance”.

Democratic donors who spoke anonymously to various media outlets were more forthright, with one calling the performance “disqualifying”. “The only way it could have been more disastrous was if he had fallen off the stage.  Big donors are saying... he has to go,” one Democratic operative told the Financial Times.

And on Friday, the New York Times editorial board called on Mr Biden to drop out. It said Democrats should “acknowledge that Mr Biden can’t continue his race, and create a process to select someone more capable to stand in his place”.

Mr Trump, meanwhile, held a rally of his own in Virginia just hours later, where he hailed a “big victory” in the debate, which CNN said was viewed by 48 million people on television and millions more online.

“Joe Biden’s problem is not his age,” the 78-year-old Trump said. “It’s his competence. He’s grossly incompetent.”

The former president said he did not believe speculation that Mr Biden would drop out of the race, saying he “does better in polls” than other Democrats, including California Governor Gavin Newsom and Vice-President Kamala Harris.

Despite concern from some media pundits, early indications suggest there has been “no change” in the polls in the aftermath of the debate, the Washington Post columnist Philip Bump told BBC R4’s Today programme.

Speaking later on the programme, pollster Frank Luntz said there may not be a significant change to voters’ intentions - as most Americans have already made up their minds about who they will vote for later this year.

Early polls do show that American voters considered Mr Trump to be the clear winner, however.

A post-debate poll by liberal pollster Data for Progress found that 62% of likely voters who watched or read about the debate found Trump won. Only 30% of those polled by the progressive polling group said Mr Biden won the debate.

Until further polling is conducted, fundraising could be another indication of continued enthusiasm for Mr Biden’s candidacy - a significant change on that front could pose a major challenge for Democrats.

In an apparent effort to show it was maintaining momentum, the Biden campaign has said that the president raised $14m from fundraisers in recent days.

And from the Thursday debate to Friday evening, it said Mr Biden had raised more than $27 million for the campaign.

Mr Biden is also expected to meet with additional donors on Saturday. He has planned fundraisers in the wealthy New York Hamptons and in Redbank, New Jersey.