WASHINGTON — President Joe Biden dropped out of the 2024 presidential race Sunday, he said in a letter posted to social media, creating an unprecedented vacancy atop the Democratic ticket one month before he was scheduled to officially accept his party’s nomination.

In a followup post less than 30 minutes later, Biden endorsed Vice President Kamala Harris to take his place as the Democratic nominee.

Biden’s withdrawal came after a weeks-long pressure campaign from party insiders following a disastrous June 27 debate performance against GOP candidate former President Donald Trump.

The move throws an already-unusual presidential race into further chaos, and it was not immediately clear Sunday how Democrats would choose a replacement for Biden in November’s election, though Harris would have a strong claim to lead the ticket.

Biden praised Harris as “an extraordinary partner” in the administration’s accomplishments.

Biden, who has been fighting a COVID-19 infection at home in Delaware since last week, was not specific about his reasons for stepping aside, but said he believed it was in the country’s best interest.

“It has been the great honor of my life to serve as your President,” he wrote in the one-page letter. “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, 81, appeared frail and confused at several points throughout the debate, leading to worries among elected Democrats and the party’s voters that he was no longer up to the task of governing or contesting Trump’s bid to win back the White House.

As several congressional Democrats called for him to quit the race, others asked that he ramp up his public schedule and include more unrehearsed appearances that could demonstrate his fitness.

But a more robust schedule of news interviews, press conferences and campaign rallies did not sufficiently quiet the Democratic voices saying Biden’s candidacy was likely to throw the presidential race to Trump – whom Biden and others have described as an existential threat to U.S. democracy – and deeply handicap Democrats in other races up and down November’s ballot.

On Friday, Sens. Sherrod Brown of Ohio and Martin Heinrich of New Mexico brought the number of senators calling on Biden to drop out to four. A day earlier, Montana Sen. Jon Tester said Biden should drop his reelection campaign and that Democrats should hold an open nomination process at their Chicago convention next month.

In the U.S. House, 29 Democrats had called for Biden to withdraw from the race by the end of the day July 19.

In a post following the announcement to his social media site, Truth Social, Trump said Biden was “never” fit to serve as president.

“Crooked Joe Biden was not fit to run for President, and is certainly not fit to serve – And never was!” Trump wrote. “He only attained the position of President by lies, Fake News, and not leaving his Basement. All those around him, including his Doctor and the Media, knew that he wasn’t capable of being President, and he wasn’t – And now, look what he’s done to our Country.”

More details of announcement

In the letter, Biden praised his administration’s accomplishments over three-and-a-half years, saying he’d worked to make “historic investments” in the country, lowered prescription drug costs, nominated the first black woman to the U.S. Supreme Court and “passed the most significant climate legislation in the history of the world.”

“Together we overcame a once in a century pandemic and the worst economic crisis since the Great Depression,”  Biden wrote. “We’ve protected and preserved our Democracy. And we’ve revitalized and strengthened our alliances around the world.”

Biden said he would “speak to the Nation later this week” about the decision.

He praised Harris and other supporters.

“For now, let me express my deepest gratitude to all those who have worked so hard to see me reelected,” he wrote. “I want to thank Vice President Kamala Harris for being an extraordinary partner in all this work. And let me express my heartfelt appreciation to the American people for the faith and trust you have placed in me.”

In follow-up posts, Biden said he was endorsing Harris and added a fundraising link.

“My very first decision as the party nominee in 2020 was to pick Kamala Harris as my Vice President,” he said. “And it’s been the best decision I’ve made. 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.”

Trump gains in polls

The about face in what was to be a 2020 presidential election rematch leaves Democrats searching for a new candidate as Trump, who promises authoritarian-style leadership, has gained support in recent polls.

With just 107 days until Election Day, Biden’s move marks the latest date in modern presidential history that a candidate has withdrawn from the race.

President Lyndon Johnson announced in March 1968 that he would not seek reelection that year, leaving Democratic delegates to decide on a replacement – ultimately Vice President Hubert Humphrey – at the party’s convention that summer in Chicago.

Harris appears to be in a strong position to replace Biden as the party’s standard bearer, though questions remain about how the process will play out and who would become the vice presidential nominee.

Democrats praise decision

Reaction poured in shortly after the Sunday afternoon announcement, with Democrats largely praising Biden’s record and calling his decision courageous.

Senate Majority Leader Chuck Schumer said in a statement that he understood Biden’s decision to step out of the race was “not easy, but he once again put his country, his party, and our future first.”

“Joe Biden has not only been a great president and a great legislative leader but he is a truly amazing human being,” the New York Democrat said.

Several Republicans called for Biden to resign his office.

“If Joe Biden is not fit to run for President, he is not fit to serve as President,” House Speaker Mike Johnson wrote on X. “He must resign the office immediately. November 5 cannot arrive soon enough.”

A crescendoing chorus to step down

Biden faced calls for him to abandon his reelection bid from congressional Democrats, even as he tried to stabilize the debate aftershock by holding a series of campaign rallies, sitting down for interviews and holding a press conference at the annual NATO conference.

Democratic lawmakers largely presented a public front of support for Biden in statements and passing interviews in the U.S. Capitol hallways with reporters.

What began as a trickle of dissent from rank-and-file Democrats — beginning with Rep. Lloyd Doggett of Texas and a handful of doubtful senior House Democrats — steadily grew to a torrent by Friday.

50-year career in Washington

Biden’s exit marks the closure of a long, storied career in Washington, including 38 years in the U.S. Senate, featuring stints leading the Foreign Affairs and Judiciary committees, and eight years as vice president under President Barack Obama.

Biden’s presidency was punctuated with major economic wins for Democrats, beginning with nearly $2 trillion to combat the fallout from the COVID-19 pandemic.

His leadership with a Democratic majority in Congress resulted in substantial nationwide infrastructure investments, drove financial incentives to tackle climate change and revive the U.S. global role in semiconductor manufacturing, and strengthened flagging tax enforcement.

However, low approval ratings followed Biden throughout his presidency as Americans aimed their frustrations over inflation at the White House and assigned blame for record numbers of border crossings as a divided Congress – after Democrats lost their House majority in the 2022 midterms – failed to pass immigration restrictions negotiated with the administration.

Biden’s handling of the Israel-Hamas war also hurt his support among young and progressive voters as Israel’s continued offensive against Hamas militants in the Palestinian territory of the Gaza Strip killed tens of thousands of civilians. Protesters against the U.S. supply of weapons to Israel interrupted dozens of Biden’s reelection campaign events through 2024.

Ariana Figueroa contributed to this report.