WASHINGTON — President Joe Biden dropped out of the 2024 race for the White House on Sunday, ending his bid for reelection after a disastrous debate with Donald Trump that raised doubts about the incumbent’s fitness for office. The unprecedented announcement, delivered less than four months before the election, immediately upended a campaign that both political parties view as the most consequential in generations.

The president — intent on serving out the remainder of his term in office — quickly endorsed Vice President Kamala Harris to take on Trump and encouraged his party to unite behind her, making her the party’s instant favorite for the nomination at its August convention in Chicago.

The announcement is the latest jolt to a tumultuous campaign for the White House, coming a week after the attempted assassination of Trump at a Pennsylvania rally.

A party’s presumptive presidential nominee has never stepped out of the race so close to the election. President Lyndon Johnson, besieged by the Vietnam War, announced in March 1968 that he would not seek another term after just a single state’s primary. Biden’s July decision comes after more than 14 million Democrats cast votes supporting him through the primary process.

Harris, in a statement, praised Biden’s ”selfless and patriotic act” and said she intends to ”earn and win” her party’s nomination.

”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,” she said.

Biden’s decision to bow out came after escalating pressure from his Democratic allies to step aside following the June 27 debate, in which the 81-year-old president trailed off, often gave nonsensical answers and failed to call out the former president’s many falsehoods.

”It has been the greatest honor of my life to serve as your President. And while it has been my intention to seek reelection, I believe it is in the best interest of my party and the country for me to stand down and to focus solely on fulfilling my duties as President for the remainder of my term,” Biden wrote in a letter posted Sunday to his X account.

Nearly 30 minutes after he delivered the news that he was folding his campaign, Biden threw his support behind Harris.

”Today I want to offer my full support and endorsement for Kamala to be the nominee of our party this year,” he said in another post on X. ”Democrats — it’s time to come together and beat Trump.”

Harris faces a staggering political to-do list in the coming days: securing the votes of delegates to claim her party’s nomination, picking a running mate, and pivoting a massive political operation that had been built to reelect Biden to boost her candidacy instead. Harris, if elected, would be the first woman and the first person of South Asian descent to be president.

There were early signs that the party was moving to coalesce around Harris, who scored the endorsements of the Congressional Black Caucus and former President Bill Clinton and former Secretary of State Hillary Clinton even before she had commented on Biden’s decision to quit the race. But notably, former President Barack Obama held off, pledging support behind the eventual party nominee.

”We are honored to join the President in endorsing Vice President Harris and will do whatever we can to support her,” the Clintons said in a statement.

But Obama, who had privately shared doubts about Biden’s reelection chances, stopped short of endorsing Harris even as he praised Biden for his decision to leave the race.

”I have extraordinary confidence that the leaders of our party will be able to create a process from which an outstanding nominee emerges,” he said in a statement.

Biden’s decision came as he has been isolating at his Delaware beach house after being diagnosed with COVID-19 last week, huddling with a shrinking circle of close confidants and family members about his political future. Biden said he would address the nation later this week to provide ”detail” about his decision.

Harris found out about Biden’s plans on Sunday morning and senior campaign and White House staff were notified just minutes before the letter went out, according to people familiar with the matter who commented on the private discussions on condition of anonymity. Biden had been reflecting on his future for the past couple days and the decision was closely held.

Now, Democrats have to urgently try to bring coherence to the nominating process in a matter of weeks and convince voters in a stunningly short amount of time that their nominee can handle the job and beat Trump. And for his part, Trump must shift his focus to a new opponent after years of training his attention on Biden.

The decision marks a swift and stunning end to Biden’s 52 years in electoral politics, as donors, lawmakers and even aides expressed to him their doubts that he could convince voters that he could plausibly handle the job for another four years.

Biden won the vast majority of delegates and every nominating contest but one, which would have made his nomination a formality. Now that he has dropped out, those delegates will be free to support another candidate.

Harris, 59, appeared to be the natural successor, in large part because she is the only candidate who can directly tap into the Biden campaign’s war chest, according to federal campaign finance rules.

Biden’s campaign formally changed its name to Harris for President, reflecting that she is inheriting his political operation — a sign of the advantage she has in the race for the Democratic nomination. Democratic groups, including the Democratic National Committee, also filed paperwork changing the names of their joint fundraising committees to reflect Harris’ candidacy.

The Democratic National Convention is scheduled to be held Aug. 19-22 in Chicago, but the party had announced it would hold a virtual roll call to formally nominate Biden before in-person proceedings begin.

It remained to be seen whether other candidates would challenge Harris for the nomination. California Gov. Gavin Newsom, who had been discussed as a possible replacement for Biden last year, and Transportation Secretary Pete Buttigieg, who challenged both Biden and Harris for the Democratic nomination in 2020, threw their support behind her Sunday.

The Democratic National Committee’s chair, Jaime Harrison, said in a statement that the party would ”undertake a transparent and orderly process” to select “a candidate who can defeat Donald Trump in November.”

West Virginia Sen. Joe Manchin, who left the Democratic Party earlier this year to become an independent, is considering re-registering as a Democrat to vie for the nomination against the vice president, according to Jonathan Kott, a longtime adviser to Manchin.

Harris was spending Sunday afternoon calling Democratic elected officials and delegates as she works to lock up the nomination.

Harris received her first delegates for the Democratic presidential nomination on Sunday. The Tennessee Democratic Party posted on X that its delegation voted during a meeting Sunday to back Harris after Biden’s departure from the campaign.

Pennsylvania Gov. Josh Shapiro, North Carolina Gov. Roy Cooper and Arizona Sen. Mark Kelly — each from a swing state and mentioned as a possible running mate for Harris — also endorsed Harris Sunday evening.

Trump reacted to the news in a post on his Truth Social site, in which he said Biden ”was not fit to run for President, and is certainly not fit to serve.”

”We will suffer greatly because of his presidency, but we will remedy the damage he has done very quickly,” he added. ”MAKE AMERICA GREAT AGAIN!”

While Trump and his team had made their preference for facing Biden clear, his campaign had nonetheless ramped up its attacks on Harris as pressure on Biden to step down intensified.

Democratic officials, including many who were behind the effort to push Biden from the race, quickly released statements praising Biden’s decision.

”His decision of course was not easy, but he once again put his country, his party, and our future first,” said Senate Majority Leader Chuck Schumer, D-N.Y. ”Joe, today shows you are a true patriot and great American.”

House Democratic leader Hakeem Jeffries of New York praised Biden as ”one of the most accomplished and consequential leaders in American history.”

Republican House Speaker Mike Johnson said Biden should immediately resign if he is not fit enough to run for office. In a statement, Johnson said, ”November 5 cannot arrive soon enough.”

In addition to his planned address to the country, Biden still intends to host Israeli Prime Minister Benjamin Netanyahu at the White House this week, according to a person familiar with the president’s schedule who was not authorized to comment publicly.

The exact timing of the meeting is still not set in stone as Biden continues to recover from COVID. Netanyahu is scheduled Wednesday to deliver an address to Congress and he is also expected to meet with Harris while in Washington.

In 2020, Biden pitched himself as a transitional figure who wanted to be a bridge to a new generation of leaders. But once he secured the job he spent decades struggling to attain, he was reluctant to part with it.

Biden was once asked whether any other Democrats could beat Trump.

”Probably 50 of them,” Biden replied. ”No, I’m not the only one who can defeat him, but I will defeat him.”

Biden is already the country’s oldest president and had insisted repeatedly that he was up for the challenge of another campaign and another term, telling voters all they had to was ”watch me.”

And watch him they did. His poor debate performance prompted a cascade of anxiety from Democrats and donors who said publicly what some had said privately for months, that they did not think he was up to the job for four more years.

Concerns over Biden’s age have dogged him since he announced he was running for reelection, though Trump is just three years younger at 78. Most Americans view the president as too old for a second term, according to an August 2023 poll from The Associated Press-NORC Center for Public Affairs Research. A majority also doubt his mental capability to be president, though that is also a weakness for Trump.

Biden often remarked that he was not as young as he used to be, doesn’t walk as easily or speak as smoothly, but that he had wisdom and decades of experience, which were worth a whole lot.

”I give you my word as a Biden. I would not be running again if I didn’t believe with all my heart and soul I can do this job,” he told supporters at a rally in North Carolina a day after the debate. ”Because, quite frankly, the stakes are too high.”

But voters had other problems with him, too — he has been deeply unpopular as a leader even as his administration steered the nation through recovery from a global pandemic, presided over a booming economy and passed major pieces of bipartisan legislation that will impact the nation for years to come. A majority of Americans disapprove of the way he’s handling his job, and he’s faced persistently low approval ratings on key issues including the economy and immigration.

Biden’s motivation for running was deeply intertwined with Trump. He had retired from public service following eight years serving as vice president under Obama and the death of his son Beau but decided to run after Trump’s comments following a ”Unite the Right” rally in Charlottesville, Virginia, in 2017, when white supremacists descended on the city to protest the removal of its Confederate memorials.

Trump said: ”You had some very bad people in the group, but you also had people that were very fine people on both sides. On both sides.”

Biden’s wife, first lady Jill Biden, responded to the president’s announcement by reposting Biden’s letter announcing his decision and adding red heart emojis.

Granddaughter Naomi Biden Neal said on social media, ”I’m nothing but proud of my Pop.” She said he has served the country ”with every bit of his soul and with unmatched distinction” and ”our world is better today in so many ways thanks to him.”

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Superville reported from Rehoboth Beach, Delaware. Associated Press writer Jill Colvin in New York and Aamer Madhani in Washington contributed to this report.

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Follow the AP’s coverage of the 2024 election at https://apnews.com/hub/election-2024.