President Joe Biden, in his first TV interview since dropping out of the White House race, warned that Republican candidate Donald Trump was "a genuine danger to American security."

"Mark my words, if he wins... this election, watch what happens," Biden told CBS News in a pre-taped interview broadcast on Sunday.

"He's a genuine danger to American security. Look, we're at an inflection point in world history... and democracy is the key."

Biden, 81, has kept a low profile since withdrawing after his flailing debate performance against Trump underlined fears about his age and mental abilities.

In the short TV interview, recorded in the White House last week, he appeared frail but cogent, again admitting he failed in the debate but stressing that health-wise 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.

"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."

Helping Harris

Biden said he was proud of his record on jobs, investment and Covid recovery -- and vowed to campaign hard for Vice President Kamala Harris, who has replaced him on the ballot.

"I'm going to do whatever Kamala thinks I can do to help most," he said.

Biden's age had come to dominate the 2024 campaign, and the Democrats' hopes of winning have soared since his exit, as Harris has enjoyed a surge in support that has left Trump struggling.

The outgoing president said he had expected to serve only one term when he won in 2020, but that he had been persuaded to stay on.

"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 didn't happen," he told CBS's Robert Costa.

Trump's light campaign 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 white supremacist attacks on his family, child care, 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. 

He 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 told CNN's Dana Bash.