He has faced dozens of calls from leading Democrats to step down after a difficult performance in a TV debate against Donald Trump last month. Tonight, Mr. Biden said in a statement that it had been the greatest honor to serve as U.S. president and that he would stay on as president for the final six months of his term.

He endorsed the vice president, Kamala Harris, to be the next Democrat nominee.

And in the last hour, she confirmed that she would be seeking the nomination. Tonight, we'll be asking what finally pushed Joe Biden out and what happens next.

But first, I'm joined by our North America editor, Sarah Smith.

Sarah, very good to have you.

What a shock moment.

What led up to this? Well, there's been near relentless pressure on Joe Biden for just over three weeks now since that disastrous debate performance against Donald Trump.

And initially, he dug his heels in pretty stubbornly and insisted that he was staying in the race and that nobody could push him out.

But the calls for him to go kept coming.

There was more and more and more Democrats coming out and saying that they thought that he should step aside.

And ultimately, it was some of his very closest allies as well, I think most notably Nancy Pelosi, who knows the Democratic Party inside out and is extremely good at counting votes as well, when she made it clear that she thought that he was in danger of losing to Donald Trump.

That put pressure on him that obviously, ultimately, he has decided there's no point in carrying on if his own party are going to be coming out against him like this.

So that, I think, is probably what led to the president's decision today. Just four days ago, Joe Biden was out campaigning for his re-election, greeting voters in Nevada.

In the days since, he's made the hardest decision of his lengthy political career, to withdraw as the Democratic presidential candidate.

Writing on social media, Catching Covid was the final nail in his political coffin.

Calls for him to step aside just kept coming from Democrats who were worried he was on course to lose the election to Donald Trump, who has just enjoyed a very successful week as he accepted his party's nomination. Today, he said, "So will Joe Biden's number two, Vice President Kamala Harris, step into first place now as the presidential candidate?" Not automatically, but he has already given her his backing.

Today, I want to offer my full support and endorsement for Kamala to be the nominee of our party this year.

Ms. Harris says she aims to earn and win the nomination and wants to unite the Democratic Party and the nation to defeat Donald Trump.

Hillary Clinton edged out Joe Biden for the party's nomination in 2016 and then went on to lose the election.

Today, both of the Clintons say they are firmly supporting Kamala Harris. But what about some of the other possible contenders?

Gretchen Whitmer, the governor of Michigan, seemed to rule herself out, saying, "While the governor of California, Gavin Newsom, said nothing about his intentions as he praised President Biden as an extraordinary history-making president." Welcome to the White House, everybody.

Joe Biden will have served 12 years in the White House altogether, eight as vice president to Barack Obama, four in the Oval Office himself.

Mr. Obama today called him one of America's most consequential presidents and a patriot of the highest order. President Biden has been recovering from COVID in the last few days at his beach house in Delaware, and our correspondent Netta Toffee is there for us now.

Netta, very good to see you.

Delaware, very much an oasis for President Biden and somewhere where he has a lot of support.

What is the mood and reaction there tonight? Yes, Sumi, where I am at one of the state park beaches is just a few hundred feet away from Joe Biden's home.

You can see the Secret Service posted behind me and outside of his private community.

They have been moving along those who have stopped to take pictures.

I can tell you that many over the last few days wondered whether Joe Biden was indeed discussing his future while he has been here at this quiet seaside town.

And so while the content of his statement may not have come as a surprise, the timing certainly has.

Many believing he would have gone to Washington, D.C., would have appeared on camera to Americans to make that statement.

Instead, many here were sunbathing when they got that news, some of them telling me it was surreal to be just a few hundred feet away from it all and to read that he had decided to stand down. I can tell you, Sumi, that across the board, whether you speak to Democrats or Republicans, they all wish President Joe Biden well, and they do feel that this announcement was inevitable, given how it appeared to so many voters that he had declined in recent months.

Nevertheless, there is some apprehension from Democrats about whether the party will be able to unite around a candidate.

They mentioned that Kamala Harris hasn't been as seen out in the public.

Her profile hasn't been built over these last few years by the administration as much as they wish it had, and questioning whether she would be able to beat former President Donald Trump.

But from Republicans, again, them saying no matter who they choose, the Democrats, they still feel Donald Trump is in a strong position.

So at this point, Sumi, all questions on where the Democratic Party goes from here.