Joe Biden has withdrawn from his presidential re-election race and endorsed Vice-President Kamala Harris to take his place at the top of their party’s ticket, an extraordinary decision upending American politics that plunges the Democratic nomination into uncertainty just months before the November election against Donald Trump – a candidate Biden has warned is an existential threat to US democracy.

“While it has been my intention to seek re-election, I believe it is in the best interest of my party and the country for me to stand down and focus solely on fulfilling my duties as president for the remainder of my term,” Biden said in a letter announcing his decision.

Biden thanked Harris in his letter and later endorsed her as the Democratic nominee for president in a tweet. He said he planned to speak to the nation in more detail later this week.

“My fellow Democrats, I have decided not to accept the nomination and to focus all my energies on my duties as president for the remainder of my term. My very first decision as the party nominee in 2020 was to pick Kamala Harris as my vice-president. And it’s been the best decision I’ve made,” he said.

“Today I want to offer my full support and endorsement for Kamala to be the nominee of our party this year. Democrats – it’s time to come together and beat Trump. Let’s do this.”

Harris thanked Biden in a statement “for his extraordinary leadership as president”. She also said “with this selfless and patriotic act, President Biden is doing what he has done throughout his life of service: putting the American people and our country above everything else.

“I am honored to have the president’s endorsement and my intention is to earn and win this nomination,” she said. “I will do everything in my power to unite the Democratic party – and unite our nation – to defeat Donald Trump and his extreme Project 2025 agenda.

“We have 107 days until Election Day. Together, we will fight. And together, we will win.”

The president made the stunning announcement after a weeks-long pressure campaign by Democratic leaders, organizers and donors who increasingly saw no path to victory so long as the embattled incumbent remained on the ticket. More than 30 Democratic members of Congress had called on Biden to step aside. As recently as Friday, his campaign had insisted he was staying in the race. An ABC News/Ipsos poll released on Sunday found that 60% of Democrats believed he should end his run. The same poll found that nearly 76% of Democrats would be satisfied with Harris as the nominee.

Biden’s decision to withdraw appears to have been abrupt. The president told his senior staff on Sunday afternoon that he had changed his mind about staying in the race, and campaign officials were still reportedly on the phone with delegates asking if they could count on their support.

In a post on Truth Social, Trump said Biden “was not fit to run for President, and is certainly not fit to serve – And never was!”

Biden “only attained the position of President by lies, Fake News, and not leaving his Basement”, Trump said. “All those around him, including his Doctor and the Media, knew that he wasn’t capable of being President, and he wasn’t.”

Trump went on to list a series of falsehoods about immigration, concluding: “We will suffer greatly because of his presidency, but we will remedy the damage he has done very quickly. MAKE AMERICA GREAT AGAIN!”

Minutes after Sunday’s announcement, Trump told CNN that he believed it would be easier to defeat Harris than it would have been to beat Biden.

It is unclear if any other Democrats will try to challenge Harris for the nomination. And it is still not clear whether she is better positioned to beat Trump. An NBC News poll from earlier this month showed Trump leading Biden and Harris by 2 points, which was within the survey’s margin of error.

Barack Obama, the former president who selected Biden as his vice-president for both of his terms, released a lengthy statement on Sunday praising Biden’s decision. There had been reporting in recent days that there was tension between the two men over Biden feeling like Obama and other Democrats were trying to push him out.

“Joe Biden has been one of America’s most consequential presidents, as well as a dear friend and partner to me,” said Obama, who won the presidency in 2008. “Today, we’ve also been reminded  –  again – that he’s a patriot of the highest order.

“I also know Joe has never backed down from a fight. For him to look at the political landscape and decide that he should pass the torch to a new nominee is surely one of the toughest in his life. But I know he wouldn’t make this decision unless he believed it was right for America. It’s a testament to Joe Biden’s love of the country – and a historic example of a genuine public servant once again putting the interests of the American people ahead of his own – that future generations of leaders will do well to follow.”

Obama, who stopped short of endorsing Harris, said Democrats would be navigating “uncharted waters in the days ahead”. He added: “But I have extraordinary confidence that the leaders of our party will be able to create a process from which an outstanding nominee emerges.”

Bill and Hillary Clinton endorsed Harris in a joint statement on Sunday. “We are honored to join the president in endorsing Vice-President Harris and will do whatever we can to support her,” the former president and secretary of state said.

Harris’s nomination is not automatic, and there are other Democrats – including the Michigan governor, Gretchen Whitmer, the California governor, Gavin Newsom, and the Illinois governor, JB Pritzker – who could seek the nomination. If any of those candidates were nominated in Chicago next month, they would face the monumental task of introducing themselves to voters, crafting a campaign message and defeating Trump – all in two and a half months.

Citing sources, CBS News on Sunday reported that neither Whitmer nor Newsom intended to pursue the Democratic nomination. The network added: “There’s no one at this moment preparing behind the scenes to challenge vice-president Harris.”

Whitmer said in a Sunday tweet: “President Biden is a great public servant who knows better than anyone what it takes to defeat Donald Trump. My job in this election will remain the same: doing everything I can to elect Democrats and stop Donald Trump, a convicted felon whose agenda of raising families’ costs, banning abortion nationwide, and abusing the power of the White House to settle his own scores is completely wrong for Michigan.”

The chairperson of the Democratic National Committee, Jaime Harrison, said the party would “undertake a transparent and orderly process to move forward” to choose a candidate to defeat Trump in November.

A disastrous debate performance last month, and his uneven public appearances since, have only exacerbated longstanding voter concerns that the 81-year-old president was simply too old to serve another four years.

Democrats immediately praised Biden’s decision, including Chuck Schumer, the majority leader in the US Senate, and one of several Democrats who had been pressuring Biden to step aside.

“Joe Biden has not only been a great president and a great legislative leader but he is a truly amazing human being. His decision of course was not easy, but he once again put his country, his party, and our future first,” Schumer said in a statement.

The US House speaker, Mike Johnson, suggested during appearances on Sunday talkshows that Republicans would bring legal challenges to attempt to block efforts to change the Democratic ticket. Experts are skeptical those efforts will succeed.

Johnson was also one of several top Republicans who called on Biden to resign the presidency – something Biden is almost certain not to do.

“If Joe Biden is not fit to run for president, he is not fit to serve as president. He must resign the office immediately,” Johnson said, adding that election day on 5 November “cannot arrive soon enough”.

The Ohio senator JD Vance, Trump’s running mate, made similar comments on Sunday.

Biden’s decision to step aside, though remain as president, caps a singular few weeks in American politics, the latest stunning episode in an unusually tumultuous election season.

Trump, the former president and Republican nominee, narrowly survived an attempt on his life during a campaign rally in Pennsylvania that bloodied his ear and left one spectator dead. Biden, after appealing for calm in the wake of attack, had returned to the campaign trail last week determined to salvage his candidacy and once again prove his doubters wrong.

In media appearances, the president was defiant, insisting that he would remain the party’s standard-bearer in November. On Wednesday, before delivering remarks at a conference in Nevada, he tested positive for Covid.

The president’s withdrawal pushes the Democratic party into largely uncharted waters, with its national convention scheduled to begin on 19 August in Chicago. The nominee will also have a tight window to choose a running mate to take on Trump and Vance. It is not clear how Democrats will choose a new ticket.

After serving as Biden’s vice-president, Harris, 59, has the largest national profile of any Democratic candidate, and delegates may view her as the safest option. Campaign finance experts also say that Harris would have the most straightforward legal argument to keep the Biden campaign’s fundraising haul, while another nominee might have to forfeit that money. As of late May, the Biden campaign had $91.6m in cash on hand.