President Joe Biden on Sunday dropped out of the 2024 presidential race and endorsed Vice President Kamala Harris as the Democratic Party's nominee, bowing to weeks of growing pressure by his own party members to quit his re-election bid against former President Donald Trump.

"It has been the greatest honor of my life to serve as your President," Biden wrote in a post on the social media site X.

"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.

"I will speak to the Nation later this week in more detail about my decision," wrote Biden, whose bombshell decision came after a sharp spike in recent days in the number of congressional Democrats publicly calling for him to step aside for another nominee.

Biden in a subsequent tweet wrote that his "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. 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," Biden wrote.

"Let's do this."

Harris in a statement said, "I am honored to have the President's endorsement and my intention is to earn and win this nomination."

Former President Bill Clinton and former Secretary of State Hillary Clinton, in a joint statement from the Democratic power couple, said, "We are honored the join the President in endorsing Vice President Harris and will do whatever we can to support her."

Biden, who has been isolated at his Rehoboth, Delaware, beach home after being diagnosed with Covid-19, and Harris spoke on Sunday before he announced he would drop out of the election race.

NBC News reported that Biden at 1:45 p.m. ET Sunday told his senior campaign team that he had changed his mind about remaining in the contest.

Trump in a phone interview with NBC News said of Biden's decision, "He should never have been there in the first place."

"He should have stayed in his basement," said Trump, who was formally nominated as the Republican Party's candidate last week, days after narrowly avoiding being killed in an assassination attempt at a Pennsylvania campaign rally on July 13.

Democratic National Committee Jamie Harrison in a statement said, "In the coming days, the Party will undertake a transparent and orderly process to move forward as a united Democratic Party with a candidate who can defeat Donald Trump in November."

"This process will be governed by established rules and procedures of the Party. Our delegates are prepared to take seriously their responsibility in swiftly delivering a candidate to the American people," Harrison said.

Biden's move echoes the decision by another Democratic incumbent, President Lyndon Johnson, to drop out of the 1968 election as he faced turmoil over the Vietnam War, low approval ratings, and a surprisingly strong performance by Sen. Eugene McCarthy in the New Hampshire primary.

Biden's campaign for weeks had insisted he would remain in the race, despite growing concern since his disastrous debate with Trump in late June that he was too old and frail to compete against the former president and serve a full second term if re-elected.

As of Sunday, nearly 40 Democratic members of Congress had urged Biden to drop out. Sen. Joe Manchin of West Virginia, who recently switched his political affiliation from Democrat to independent, earlier Sunday publicly said Biden should quit the race.

Rep. Nancy Pelosi, the California Democrat who previously served as speaker of the House of Representatives, in a tweet Sunday wrote, "President Joe Biden is a patriotic American who has always put our country first."

"His legacy of vision, values and leadership make him one of the most consequential Presidents in American history," Pelosi wrote. "With love and gratitude to President Biden for always believing in the promise of America and giving people the opportunity to reach their fulfillment. God blessed America with Joe Biden’s greatness and goodness.

Biden's decision to drop out blows open the presidential race less than four months before Election Day.

It also poses extraordinary questions and challenges for the Democratic party, which now must scramble to assemble a new presidential ticket.

The next candidate will need to compete against the Republican ticket of Trump and Sen. JD Vance, win over donors, delegates and other allies who were loyal to the incumbent — all while convincing voters of their own worthiness for the nation's highest office.

Biden's campaign did not immediately respond to a request for comment.

Biden started to hemorrhage support from his own party and allies after his June 27 debate against Trump.

While he is only three years older than Trump, Biden looked and sounded significantly worse on the debate stage. He displayed a stiff posture and sometimes vacant-looking gaze, and he frequently gave answers that were raspy, unclear and ineffective.

In both substance and appearance, Biden contrasted sharply with Trump, who looked and sounded the same as he has in recent years.

The debate instantly set off a crisis among Democrats. Even before it ended, party operatives were raising doubts about whether Biden could effectively campaign against Trump.

High-profile Democratic donors and backers openly urged Biden to drop out, with some vowing to withhold campaign contributions in the meantime.

Biden and his aides spent weeks trying to stanch the bleeding.

In his first post-debate interview, Biden said he was convinced that he is the best person to take on Trump, adding that he could only be persuaded to step down "if the Lord Almighty comes out and tells me that."

But as his resolute posture failed to quell the unrest in his party, Biden suggested he could be moved to withdraw in other ways.

In a BET interview that aired Wednesday during the Republican National Convention, Biden said he might reconsider his decision to stay in the race "if I had some medical condition that emerged."

On the same day that interview was set for broadcast, Biden tested positive for Covid. He canceled a scheduled appearance in Las Vegas and returned his beach house in Delaware.

Biden is already the oldest person ever to serve in the White House.

He would have been 86 at the end of a second term. He had weathered questions about his physical and mental fitness even during his 2020 run.

Those concerns grew in the 2024 cycle, but they were aggressively batted down by Biden's campaign and White House aides, and the president's relative lack of press conferences and interviews may have shielded scrutiny about his acuity.