In the short television interview, recorded in the White House last week, he appeared frail but cogent, again admitting he failed in the debate but stressing that healthwise he has “no serious problem”.

Explaining his exit, he said other Democratic Party politicians standing for re-election feared he would damage their chances – and added that his only priority was to stop Trump from returning to power.

“A number of my Democratic colleagues in the House and Senate thought that I was going to hurt them in the [election] races,” he said.

“I was concerned if I stayed in the race, that would be the topic you’d be interviewing me about,” he continued.

Biden singled out former House Speaker Nancy Pelosi, a party heavyweight whose refusal to explicitly back his campaign was seen by many as pivotal.

“You’d be interviewing me about why did Nancy Pelosi say [something] … I thought it’d be a real distraction,” the president said.

“A critical issue for me still is – not a joke – maintaining this democracy.

“I have an obligation to the country to do what is the most important thing we can do, and that is – we must, we must, we must defeat Trump.”

“I’m going to do whatever Kamala thinks I can do to help most,” he said.

The outgoing president said he had expected to serve only one term when he won in 2020, but that he had been persuaded to push for a second.

“I thought of myself as being a transition president – I can’t even say how old I am. It’s hard for me to get it out of my mouth – but things got moving so quickly, it did not happen,” he told CBS’ Robert Costa.

As Harris holds huge rallies in swing states, Trump’s light schedule has come under scrutiny, and it was his running mate, J.D. Vance, who blitzed the Sunday morning political talk shows.

Appearing on CNN, ABC and CBS, Vance fielded questions about childcare, asylum seekers and abortion.

In one testy exchange with CBS’s Margaret Brennan, Vance complained that she had asked “six questions about abortion”.

“I’m still trying to get a clear answer,” Brennan retorted.

Vance also claimed that Harris was the one “calling the shots” in the Biden administration.

“If she’s not calling the shots, Dana, who is?” he asked CNN’s Dana Bash.

02:07

US vice-presidential hopeful Tim Walz slammed by Trump supporters for China connection

US vice-presidential hopeful Tim Walz slammed by Trump supporters for China connection

Meanwhile, Trump reportedly has used a slur often targeted at women to describe Harris during at least two private conversations. His campaign denies it.

The New York Times cited two people who, on different occasions, heard Trump call Harris a “b****”. The people were granted anonymity to describe private discussions.

In response, Trump campaign spokesman Steven Cheung said: “That is not language President Trump has used to describe Kamala and it’s not how the campaign would characterise her.”

Trump has a history of making derogatory statements about women and his political opponents. He has called Harris and other women, including 2016 Democratic nominee Hillary Clinton, “nasty”, and he bragged about grabbing women’s genitalia in a controversial Access Hollywood tape.

He called Carly Fiorina, one of his rivals for the Republican nomination in 2016, “horseface” during a campaign debate. Last month, Trump said falsely that Harris, who is black and of Asian descent, has misled voters about her race.

Trump has also directed disparaging comments towards men. He branded former rivals Senator Marco Rubio of Florida “liddle Marco”, Senator Ted Cruz of Texas “lyin’ Ted” and former Governor Jeb Bush of Florida “low-energy Jeb”.

Additional reporting by Associated Press