President Biden has said he's not going anywhere after phoning into a morning TV show in America. It comes after a growing number of senior members of his Democratic Party have privately joined calls for him to pull out of the presidential race after a disastrous performance in a TV debate with Donald Trump. Our North America editor, Sarah Smith, sent this report.

Seeking salvation for his presidential campaign. Joe Biden started at a church in Philadelphia yesterday and followed up with a busy schedule of events, specifically designed to show he's got the stamina required for this election. For more years, for more years. And to showcase some of his biggest supporters.

This is the President. This is our guy. This is our nominee. And he is entitled to take this to November because he's been a great President, and he's the only person that's ever beaten Trump.

But as Democrats come back to Capitol Hill today after a holiday break, it's expected more of them will call for Joe Biden to step aside. This morning, he sent a letter telling them he's not backing down, saying, I am firmly committed to staying in this race, to running this race to the end, and to beating Donald Trump. And he called into our Breakfast TV show to repeat the same message.

And I've not lost. I haven't lost. I beat him last time. I'll beat him this time.

And he dared any potential rivals to try to topple him.

But many of these guys don't think I should run against me. Announce the President. Challenge me at the convention.

It will take more than this, though, to erase memories of that disastrous debate performance.

Dealing with everything we have to do with... Look, if we finally beat Medicare- Thank you, President.

I hear you.

Guys, the White House spokeswoman faced a barrage of questions about why an expert on Parkinson's disease has visited the White House eight times in the last year.

Has the President been treated for Parkinson's? No. Is he being treated for Parkinson's? No, he's not. Is he taking medication for Parkinson's? No. Those are the things that I can give you full-blown answers on, but I'm not going to confirm a specialist, any specialist that comes to the White House out of privacy.

In a primetime interview with ABC News on Friday, he said he didn't need to take a cognitive test and that no one could persuade him to stand down as a candidate.

No one said, none of the people said, I should leave. But if they do? Well, we're not going to do that. You sure? Yeah, I'm sure. Look, I mean, if the Lord Almighty came out and said, Joe, get out of the race, I get out of the race. The Lord Almighty is not coming down.

Vice President Kamala Harris has been steadfastly loyal, as those who want Mr. Biden to step down are uniting around the idea that she should be the one to take over. No one can force him out of the race, but many will keep trying to persuade him to go. Sarah Smith, BBC News, Washington.