Mr Biden, 81, said in a Sunday written statement that it was the “greatest honour” to serve but his withdrawal was “in the best interest of my party and the country”. The announcement caps a tumultuous period in US politics, which began with his sometimes incoherent debate performance against Donald Trump on 27 June. Mr Biden says he will remain president until January.

Ms Harris, 59, said that she was “honoured” to be endorsed, adding she would “earn and win this nomination” and unite the country against Trump. “We have 107 days until election day,” she said. “Together, we will fight. And together, we will win.”

While Ms Harris has been picking up endorsements from many big figures in the party, she is yet to be officially nominated, and that may not happen until the Democratic National Convention in August. A resurgent Trump meanwhile has pulled ahead in polling and was confirmed as Republican nominee at the party’s convention in Milwaukee this week, five days after surviving an assassination attempt.

In the wake of Mr Biden’s decision, he declared the president “was not fit to run… and is certainly not fit to serve”. Other senior Republicans joined him in their criticism, and called on Mr Biden to leave the White House immediately, not just the Democratic candidacy.

• LIVE UPDATES: Joe Biden withdraws from US presidential race

Sources told the BBC that even senior White House staff and campaign officials were told of Mr Biden’s decision only moments before the statement was released on Sunday afternoon, although the president had spoken to Ms Harris and a handful of others beforehand.

Dozens of senior Democrats and grandees including former president Barack Obama, Senate leader Chuck Schumer and former House speaker Nancy Pelosi immediately praised the decision and lauded Mr Biden’s accomplishments in office.

Former president Bill Clinton and 2016 Democratic candidate for president Hillary Clinton said they backed Kamala Harris as the party’s candidate for November’s vote, saying they would “fight with everything we’ve got to elect her”.

While Mr Obama stated that he had “extraordinary confidence” that an “outstanding nominee emerges”, he did not explicitly back Ms Harris or any other candidate.

Ms Pelosi has not commented. Peter Welch, the first Democratic senator to call on Biden to drop his re-election run, called for an “open process” to nominate Harris. But there are already signs that many in the party will unify behind her, including from high-profile politicians who had been touted as potential rivals for the nomination in the event Mr Biden stepped aside.

California Governor Gavin Newsom, who is believed to have presidential ambitions, praised Mr Biden as “selfless” and said he backed the “fearless” and “tenacious” Ms Harris to face Trump. Josh Shapiro, the Pennsylvania governor, said he would do “everything I can to help elect Kamala Harris as the 47th President of the United States”.

Pete Buttigieg, the current transport secretary and a former presidential contender, said Mr Biden was “one of the most consequential presidents in American history”, adding he would do “all that I can to help elect Kamala Harris the next President.”

Gretchen Whitmer, the governor of Michigan, stated that her job “will remain the same… doing everything I can to elect Democrats and stop Donald Trump.” The Democratic National Committee meanwhile filed to amend the names of its fundraising committees to the Harris Victory Fund and Harris Action Fund.

Two major Democratic donors – LinkedIn co-funder Reid Hoffman and investor Alexander Soros – publicly endorsed Harris.

And within an hour of Mr Biden’s announcement, the pro-Trump super-PAC campaign fund Make America Great Again posted an advert attacking Ms Harris, claiming “she covered up Joe’s obvious mental decline”.

Trump added: “Whoever the Left puts up now will just be more of the same.”

Mr Biden had faced intense scrutiny since his debate performance in June. Less than two weeks ago, he hosted a high-profile summit with Nato leaders in Washington.

The occasion did little to calm nerves within his own party, with him mistakenly introducing Ukrainian President Volodymyr Zelensky as “President Putin”, and appearing to refer to Ms Harris as “Vice President Trump”.

At one point he told an interviewer that only the “Lord Almighty” could make him withdraw, but then later said he would consider doing so if he had a health condition. On Friday, while in isolation after testing positive for Covid, he said he would return to the campaign trail in the coming week.

In his statement on Sunday, Mr Biden thanked his Ms Harris, saying she was an “extraordinary partner”.

“And let me express my heartfelt appreciation for the American people for the faith and trust you have placed in me,” his statement added.

“I believe today and always have: that there is nothing America can’t do – when we do it together. We just have to remember we are the United States of America.”

Officials from the Democratic National Committee held an emergency meeting on Sunday evening.

The focus will now be on the party’s national convention, which is scheduled to start on 19 August.

Mr Biden swept the party’s primaries, meaning that the delegates representing each state at the convention were pledged to vote for him – although they are now expected to be released to vote for another candidate.