WASHINGTON — President Joe Biden announced Sunday that he will end his presidential re-election campaign, bringing an abrupt and humbling conclusion to his half-century-long political career and scrambling the race for the White House just four months before Election Day.

Biden, 81, could not reverse growing sentiment within his party that he was too frail to serve and destined to lose to Donald Trump in November. He backed Vice President Kamala Harris to replace him as the Democratic nominee.

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 on X. “I will speak to the Nation later this week in more detail about my decision.”

Biden thanked Harris for “being an extraordinary partner” in his letter and then endorsed her in a subsequent post.

“My very first decision as the party nominee in 2020 was to pick Kamala Harris as my Vice President,” Biden posted. “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.

Biden and Harris spoke Sunday ahead of the president’s announcement, according to a source familiar with the campaign. Harris did not immediately comment after the announcement.

Biden — who’d for weeks insisted publicly that he planned to stay in the race — told his senior team he had changed his mind about continuing his campaign at 1:45 p.m. ET, according to a source with direct knowledge. The tweet announcing his decision was sent at 1:46 p.m.

His withdrawal caps a singular national political career, bookended by Richard Nixon’s fall and Trump’s rise. He mounted four presidential bids. He spent 36 years in the U.S. Senate representing tiny Delaware. He rose to the chairmanships of the powerful Judiciary and Foreign Relations committees. And he served eight years as Barack Obama’s vice president.

‘Great public servant’

Reactions from politicians quickly began pouring in.

California Gov. Gavin Newsom wrote in a post on X that Biden “has been an extraordinary, history-making president — a leader who has fought hard for working people and delivered astonishing results for all Americans.”

“He will go down in history as one of the most impactful and selfless presidents,” said Newsom, who was one of the most prominent Biden surrogates. Newsom has also been floated as a possible Democratic presidential contender.

Michigan Gov. Gretchen Whitmer, another prominent Democrat talked about as a future national candidate, called Biden a “great public servant” in a post on X.

“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,” Whitmer wrote.

The president’s granddaughter Naomi Biden said in a post that she was “nothing but proud today of my Pop,” adding that he has “served our country with every bit of his soul and with unmatched distinction.” First lady Jill Biden posted her husband’s statement to X with a heart emoji.

Across the aisle, Republicans slammed the move and many called on him to resign from office, days after the completion of a Republican National Convention where speaker after speaker slammed the Biden-Harris ticket.

“If Joe Biden is not fit to run for president, he is not fit to serve as President. He must resign the office immediately,” said House Speaker Mike Johnson.

In a brief phone interview with NBC News, Trump reacted to Biden’s decision, calling the president “the worst president in the history of the United States by far.”

When asked whether he was surprised by Biden’s decision, Trump said that Biden “should never have been there in the first place.”

“He should have stayed in his basement,” Trump said.

In a fundraising email, Trump’s campaign said that Biden “quit the race in complete disgrace.”

An unprecedented decision

Biden’s decision to exit the race less than a month before his party’s convention and a few months before voters head to the polls is unprecedented in the modern political era. The last sitting president to abandon a re-election bid was Lyndon Johnson, whose expansion of the Vietnam War in the 1960s split the Democratic Party. But Johnson’s announcement came in March 1968 — eight months before that election.

“We’re in uncharted waters,” said Barbara Perry, a presidential studies professor at the University of Virginia’s Miller Center. “No president has dropped out or died this close to the convention.

Replacing Biden atop the Democratic ticket is likely to set off internal Democratic tremors as ambitious officials maneuver to become his successor. Factions have already formed around Harris, Whitmer and Newsom.

Harris, who at 59 is 22 years younger than Biden, would seem to be the heir apparent. She broke a barrier as the first female vice president. A woman of color, she enjoys strong support among African Americans, a loyal piece of the Democratic coalition. Overall, though, Harris’ approval rating stood at only 32% in an NBC News poll released earlier this month.

“There’s no one you can name right now who is an obvious substitute,” Perry said. “That’s what makes this so uncertain and chaotic.”

Unlike Republican delegates, who are bound to their candidate, Democratic delegates aren’t, so they are free to do what they want at the convention. Biden could have some influence over the delegates, but they could vote for a different candidate than his chosen pick, pending any rule changes at the convention meant to address this unprecedented situation. The rules currently say that the delegates simply have to “in all good conscience reflect the sentiments of those who elected them.”

Democratic National Committee chair Jaime Harrison said in a statement: “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.”

The mechanics of putting a new name on ballots also gives rise to myriad legal questions. Republicans could work to throw obstacles in the nominee’s path by mounting legal challenges aimed at keeping that person off the ballot.

Johnson, the House speaker, said in an interview on CNN Sunday before Biden’s announcement that “I think they have got legal hurdles in some of these states, and it’ll be litigated.”

“They have got a real problem,” he added.

Some Democrats quickly backed Harris.

Congressional Black Caucus chairman Rep. Steven Horsford, D-Nev., and Rep. Gregory Meeks, D-N.Y., the chair of the group’s PAC, said they join Biden in “fully supporting” Harris to be the Democratic presidential nominee.

“I know one thing, I will only work for @KamalaHarris!” Rep. Jasmine Crockett, D-Texas, said in a post on X shortly before Biden endorsed Harris. “If it’s anyone OTHER than her, enjoy campaign season.”

Sen. Mazie Hirono, D-Hawaii, said in a statement that “I look forward to doing everything in my power to get her elected to the White House, so we can continue building on the progress of the last four years.”

Former President Barack Obama did not immediately endorse Harris. “We will be navigating uncharted waters in the days ahead. But 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.

Debate spurred calls for resignation

Questions about Biden’s capacities dogged him throughout his presidency, but peaked following his debate with Trump on June 27. Combined with his flagging poll numbers, his listless performance sparked a panic among his own party that he couldn’t win in November.

With 51 million people watching, Biden spoke in a raspy voice and often failed to complete thoughts or deliver a cogent explanation of why voters should choose him over Trump. He later attributed his poor performance to exhaustion and a cold. He implored the country not to let one bad night overshadow his accomplishments in office.

Unpersuaded, Democratic lawmakers began calling on him to step aside, a rebellion that started slowly but grew steadily in size and intensity. Thirty-seven congressional Democrats, including independent Sen. Joe Manchin, who previously was in the Democratic Party, had called on Biden to drop out of the 2024 election before he delivered his decision Sunday afternoon.

They appealed to Biden’s patriotism, arguing that if he sincerely believed Trump is a threat to democracy, he needed to put his country first and stand down.

Biden fought back. He held numerous calls and meetings with Democratic officials at all levels to shore up support inside the party — to no avail.

Looking to blunt concerns about his acuity, he gave interviews and held press conferences to prove to voters that he could still think on his feet. But the gaffes kept coming and his poll numbers remained stagnant.

In another bit of bad fortune and timing, Biden tested positive for Covid-19 on July 17, forcing him off the campaign trail.

For Democrats, Biden’s illness created an unwanted contrast. While Trump delivered a triumphal speech accepting the Republican nomination in Milwaukee on July 18, five days after surviving an assassination attempt, Biden was in self-isolation back home.

Trump rebound

Biden’s departure is the latest in a series of jarring developments that has made the 2024 presidential race the most chaotic in living memory. Trump easily won the Republican presidential nomination despite splitting his time between the campaign trail and various courtrooms where he was a defendant in criminal and civil cases. In May, a jury in Manhattan convicted him on 34 felony counts related to hush money payments to a porn star.

Then, in short order, Trump rebounded. The Supreme Court issued a much-anticipated ruling on July 1 that immunized Trump from official conduct when he was president, impeding special counsel Jack Smith’s efforts to prosecute Trump for interfering in the 2020 election.

Trump nearly died on July 13 as he appeared at a rally in Butler, Pennsylvania. A gunman lying on a roof 130 yards away fired a bullet that grazed his ear. Trump dropped to the ground in self-defense. Then, his face smeared with blood, he rose and defiantly pumped his fist, yelling “Fight!”

Another fortuitous development came two days later, coinciding with the opening of the Republican convention in Milwaukee that certified Trump’s nomination. Federal Judge Aileen Cannon in Florida dismissed a separate case brought by Smith alleging that Trump improperly retained classified documents that he took home with him when he left the White House in 2021. Cannon, who had been appointed by Trump, ruled that Smith’s appointment was illegal. Smith quickly appealed her ruling.

The rolling streak of news gave Trump a jolt of momentum, allowing Republicans to present an energized and unified front at this month’s convention.

Biden seldom sparked any such enthusiasm. HIs primary campaign was mostly a coronation. He faced token opposition as party leaders cleared the field, betting that having beaten Trump once before, Biden was best positioned to do it again. But poll after poll confirmed that voters harbored doubts about him, believing he was too old and infirm to serve another term.

An AP-NORC survey released July 17 found that a whopping 65% of Democrats believed Biden should exit the race.

Pressed by his voters, abandoned by party leaders, Biden gave in