Joe Biden has dropped out of the race, upping the race for the White House

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

Mr Biden, 81, said in a written statement on Sunday that it had been a “great honour” to serve but that he was stepping down “in the best interest of my party and the country”.

The announcement caps a tumultuous period in US politics that began with his sometimes incoherent debate show against Donald Trump on June 27. Mr Biden says he will remain president until January.

Ms Harris, 59, said she was “honoured” to be recognized and said she would “earn and win this nomination” and unite the country against Trump.

He said there are 107 days left for the election day. “Together, we will fight. Together, we will win.”

Although Ms. Harris has received endorsements from several bigwigs in the party, she has yet to be officially nominated, and that won’t happen until the Democratic National Convention in August.

Meanwhile, a resurgent Trump surged in the polls and was confirmed as the 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 “unfit to run … and certainly not fit to serve”. Other senior Republicans joined him in their criticism, calling on Mr Biden not only to seek the Democratic nomination but also to leave the White House immediately.

Sources told the BBC that senior White House staff and campaign officials were told of Mr Biden’s decision minutes before the statement was issued on Sunday afternoon, although the president had already spoken to Ms Harris and a few others.

Dozens of senior Democrats and senators, including former President Barack Obama, Senate President Chuck Schumer and former House Speaker Nancy Pelosi, immediately praised the decision and praised Mr Biden’s achievements.

Former President Bill Clinton and 2016 Democratic presidential nominee Hillary Clinton said they would support Kamala Harris as the party’s nominee for the November ballot, saying they would “fight with everything we’ve got to get her elected.”

Although Mr Obama said he had “extraordinary confidence” that a “great candidate would emerge”, he did not publicly endorse Ms Harris or any other candidate.

Ms. Pelosi did not comment.

Peter Welch, the first Democratic senator to call on Biden to drop his re-election bid, called for an “open process” to nominate Harris.

But there are already signs that many in the party will unite behind him, including top politicians who have been tipped as potential contenders for the nomination in the event Mr Biden withdraws.

California Governor Gavin Newsom, who is believed to have presidential ambitions, hailed Mr Biden as “selfless” and said he supported Ms Harris who was “fearless” and “determined” to take on Mr Trump.

Pennsylvania Governor Josh Shapiro said he will do everything in his power to elect Kamala Harris as the 47th president of the United States.

Pete Buttigieg, the current transportation secretary and former presidential contender, called Mr. Biden “one of the most important presidents in American history” and added that “I will do everything I can to elect Kamala Harris as the next president.”

Gretchen Whitmer, the governor of Michigan, said her mission “will remain the same … to do everything I can to elect Democrats and stop Donald Trump.”

The Democratic National Committee, meanwhile, filed to change the names of its fundraising groups to the Harris Victory Fund and the Harris Action Fund.

Two major Democratic donors — LinkedIn co-funder Reed Hoffman and investor Alexander Soros — publicly supported Harris.

Within an hour of Mr Biden’s announcement, the pro-Trump super-PAC campaign fund Make America Great Again published an ad attacking Mr Harris, saying he “covered up Joe’s obvious depression”..

Trump added: “Whoever the left is putting forward now, they’ll be more.”

Mr Biden has come under intense scrutiny since his debate show in June. Two weeks ago, he held a high-level summit with NATO leaders in Washington.

At one point he said in an interview that only “God Almighty” could take him back, but later said he would consider doing so if his health improved. On Friday, while in self-isolation after testing positive for Covid, he said he would be back on the campaign trail in the coming week.

In his statement on Sunday, Mr Biden thanked his wife Harris, calling her an “extraordinary partner”.

“I express my sincere appreciation to the American people for the trust and confidence you have placed in me,” his statement added.

“I believe today and always have: America can’t do anything — when we do it together. We must remember that we are America.”

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

The focus is now on the party’s national convention, which begins on August 19.

Mr Biden won the party primaries, meaning delegates representing each state at the convention pledged to vote for him – although they are expected to be freed up to vote for another candidate.