US President Joe Biden has ended his re-election campaign and endorsed Vice-President Kamala Harris to succeed him as the Democratic candidate, in an extraordinary decision that upends an already dramatic race for the White House.

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 Donald Trump.

“We have 107 days until election day,” she said. “Together, we will fight. And together, we will win.”

However, several other senior Democrats are also being touted as replacements, and a choice must be made at the party’s national convention in Chicago in August.

If the party struggles to unify around Ms Harris, the battle to succeed Mr Biden could play out on the convention floor.

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.

Trump said on Sunday that 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.

Party sources told the BBC that White House staff were told only moments before the statement was released on Sunday afternoon, although Mr Biden had spoken to Ms Harris beforehand.

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

However, not all have openly accepted Mr Biden’s endorsement of Ms Harris.

Mr Obama stated that he had “extraordinary confidence” that an “outstanding nominee emerges” but did not explicitly back Ms Harris. 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.

Former president Bill Clinton and 2016 Democratic candidate for president Hillary Clinton said they backed her as a candidate, saying they would “fight with everything we’ve got to elect her”.

Several of those who could challenge for the nomination also praised Mr Biden – but stopped short of commenting on his endorsement of his vice-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.”

Gavin Newsom, California’s governor, praised Mr Biden as a “selfless” president, but stopped short of further comment on who would succeed him.

Pete Buttigieg, the current transport secretary, said Mr Biden had “earned his place among the best and most consequential presidents in American history”. He too did not address the next step for the party.

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.

Mr Hoffman said on X/Twitter that when “presented with the choice between Kamala Harris and Donald Trump, I believe in the American people to make the right decision for our country”.

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

Donald 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 Ms Harris as “Vice President Trump”.

At one point he told an interview that only the “Lord Almighty” could make him withdraw, but then later said he would consider withdrawing if he had a health condition. On Friday, while in isolation after testing positive for Covid, he pledged to 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.