REHOBOTH BEACH, Del. — President Biden abruptly ended his reelection campaign Sunday, sending shock waves through the political world and plunging the Democratic Party into an unprecedented scramble to choose a new nominee to face former president Donald Trump.

“It has been the greatest honor of my life to serve as your President,” Biden, 81, wrote in a letter he posted to social media Sunday afternoon. “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.”

In a separate social media post Sunday, Biden endorsed his vice president, Kamala D. Harris, to replace him as the Democratic Party’s standard-bearer ahead of its national convention Aug. 19-22.

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

Biden’s exit leaves his party in an almost unprecedented position just months ahead of the Nov. 5 election. In a presidential race that has already been rocked by a felony conviction and an assassination attempt, the latest plot twist added to the sense of tumult that has gripped the nation’s politics this year.

Even as the party appeared to be coalescing around Harris on Sunday, some lawmakers were advocating for a more open process to allow other potential candidates to compete for the nomination, further highlighting the turbulent impact of Biden’s decision. Harris, a former senator from California, would bring her own liabilities to the race against Trump, whom Democrats consider an existential threat to democracy. Her approval ratings have largely mirrored the decline of Biden’s since 2021, and her campaign in the 2020 presidential primary fell apart before voting began.

Notably, former president Barack Obama did not endorse Harris in his statement, which praised Biden and suggested that Democrats would ultimately find “an outstanding nominee.”

In a statement Sunday, Harris said she would be seeking to “unite the Democratic Party” and win the presidency.

“I am honored to have the President’s endorsement and my intention is to earn and win this nomination,” she said.

Harris, 59, is the first woman, Black person and Asian American to serve as vice president, and now she has the opportunity to become the country’s first female president. She received a rapid flow of endorsements Sunday from top Democrats, including dozens of lawmakers, multiple governors, key interest groups like the Service Employees International Union, former president Bill Clinton and former secretary of state Hillary Clinton.

Biden’s allies and critics responded to his announcement by publicly gearing themselves up for a new, unpredictable battle for the White House.

Senate Majority Leader Charles E. Schumer (D-N.Y.), who had privately told Biden that his continued candidacy would harm Democrats’ chances, was among dozens of elected Democrats who praised Biden for stepping aside.

“Joe Biden has not only been a great president and a great legislative leader but he is a truly amazing human being,” Schumer said in an emailed statement, which did not endorse Harris.

Trump attacked Biden on Sunday and pledged to undo much of his legacy in a post to his social media platform, Truth Social.

“Crooked Joe Biden was not fit to run for President, and is certainly not fit to serve — And never was!” Trump wrote. “We will suffer greatly because of his presidency, but we will remedy the damage he has done very quickly.”

Several Republican lawmakers, including House Speaker Mike Johnson (La.), called on Biden to resign the presidency immediately, arguing that if he could not continue his campaign, he should not remain in office. In his statement, Biden made clear that he plans to serve out his term.

Biden’s announcement came as a shock to many of his aides and advisers, some of whom had appeared on television Sunday morning and insisted he was staying in the race. Biden, who is isolating with a case of covid, informed Harris, his chief of staff Jeff Zients and campaign chair Jen O’Malley Dillon of his decision on Sunday afternoon, according to a person familiar with the matter who spoke on the condition of anonymity to describe private discussions. He told his senior White House staff of the decision at 1:45 p.m. The president’s statement was posted to X at 1:46 p.m.

“Last night, it was full speed ahead,” the person said, adding that Biden finalized his decision late Saturday with a small group of close aides.

Before Biden’s announcement, Harris’s staff had started preparing for her debate with Republican vice-presidential nominee J.D. Vance, according to two people who spoke on the condition of anonymity to discuss a confidential process. Biden and Trump were scheduled to face off in a second debate on Sept. 10. It is unclear if Trump and Harris will do so. If Harris secures the nomination, she will have to select a running mate under a compressed timeline to ensure the Democratic ticket meets all state ballot deadlines.

Before any of that could happen, Biden’s campaign apparatus began the process Sunday of morphing into a “Harris for President,” campaign, a process that will involve creating new iconography, reengaging donors and deploying Biden’s considerable campaign war chest to introduce Harris to voters.

Biden’s campaign, headquartered in Wilmington, Del., held a call Sunday to let staff know that they were still employed and would continue their long-held mission to defeat Trump in November. O’Malley Dillon and campaign manager Julie Chavez Rodriguez led the call.

The Democrats’ predicament — a candidate dropping out after sweeping mostly unchallenged through the party primaries to become the presumptive nominee — is untested in the modern era. It caps weeks of delicate strategizing by party leaders on how to dislodge Biden, a proudly stubborn figure known to bristle at those who write him off. It signals the conclusion of a remarkable half-century political career that began when Biden won election to the Senate in 1972 as one of the youngest-ever senators and will now conclude in January with his service as the oldest-ever president.

The Democrats’ process for choosing their nominee will be hurried and fraught with deep uncertainty and the potential for further intraparty turmoil. The Democratic National Convention is in four weeks in Chicago, although Democrats had initially planned to formally nominate Biden in a “virtual roll call” before the in-person gathering.Seeking to head off any sense of disunity, multiple state delegations to the Democratic National Convention said Sunday they would unanimously back Harris, giving her a jumpstart to securing the nomination.

The president will leave office with notable accomplishments, especially for a one-term president in an era of deep division. He pushed through bills on infrastructure, climate change, health care, gun control and the semiconductor industry. He pulled the United States out of Afghanistan, rebuilt American alliances and led a coalition to defend Ukraine against Russia.

At the same time, however, he suffered historically low approval ratings and was blamed for stubbornly high inflation, record migration at the U.S.-Mexico border and a chaotic withdrawal of troops from Afghanistan in 2021.

His staunch support for Israel in the Gaza war sparked condemnation at home and abroad, and lingering concerns over his advanced age drove down his poll numbers, leaving him trailing Trump in many crucial swing states.

In abandoning his reelection campaign, Biden joins two other incumbent presidents in modern history who chose not to seek reelection: President Harry S. Truman in 1952 and President Lyndon B. Johnson in 1968. Those presidents’ announcements, however, came months earlier and gave their party far more time to regroup for the general election. Even so, Democrats lost both elections.

Many Democrats considered Biden’s June 27 presidential debate against Trump shocking and catastrophic, as Biden sometimes struggled to complete sentences or marshal his thoughts. The president and his aides insistedfor weeks afterward that he was staying in the race, likening the moment to other occasions in his long career when he had been counted out.

Voters have long told pollsters they were concerned about Biden’s age andthe debate was an inflection point, releasing a wave of Democratic anxiety that the Biden campaign had previously been able to keep in check.

On July 2, Rep. Lloyd Doggett (D-Tex.) became the first House Democrat to publicly call on Biden to step aside as the nominee, invoking Johnson’s decision 56 years earlier. “Under very different circumstances, he made the painful decision to withdraw,” Doggett said. “President Biden should do the same.”

That kicked off an agonizing stretch for Democrats when every day or so, another congressional Democrat would issue a statement declaring their affection for Biden and admiration for his accomplishments, but adding that it was time for him to “pass the torch.”

A native of Scranton, Pa., Biden entered politics more than five decades ago, winning a seat on the New Castle County Council in Delaware in 1970. He first ran for Senate in 1972, defeating a Republican incumbent, J. Caleb Boggs, in a scrappy, underdog campaign.

Tragedy struck Biden’s life within weeks of his Senate election, when his wife, Neilia, and 1-year-old daughter, Naomi, were killed in a car accident while Christmas shopping in Delaware. His two young sons, Beau and Hunter, who were also in the car, were seriously injured. Biden pushed ahead, plunging into life in the Senate, raising his boys and getting remarried to his current wife, Jill — the first chapter in a long biography of tragedy and recovery.

As Obama’s presidency wrapped up in 2016, Biden, who served as his vice president, agonized over whether to run to succeed him, but he ultimately opted against it as he continued to grieve after another personal tragedy: His elder son, Beau, had been diagnosed with brain cancer and died in 2015 at age 46.

Trump then shocked the political world by defeating Hillary Clinton for the presidency, sending the Democrats into a brooding exile and dramatically raising the stakes for the 2020 campaign. Biden later said he was inspired to join that race after watching Trump decline to unequivocally condemn a white supremacist rally in Charlottesville in 2017. He won his primary and defeated Trump in a year when the campaign was upended by the coronavirus pandemic.

Running for a second term, Biden pitched himself as uniquely equipped to defeat Trump after doing so in 2020, facing minimal opposition in the Democratic primaries even as voter concerns lingered about his age. His campaign focused heavily on abortion rights, promising to restore the federal right to an abortion after the Supreme Court overturned Roe v. Wade in 2022.

“I will speak to the nation later this week in more detail about my decision,” Biden wrote in his Sunday letter announcing his withdrawal from the race.

Svitek reported from Washington. Liz Goodwin, Tyler Pager and Leigh Ann Caldwell contributed to this report.