President Joe Biden announced on 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 re-election, 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 today ahead of the president’s announcement, according to a source familiar with the campaign.

A Singular National Political Career

Biden’s 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.

Reactions Pour In

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.” Michigan Gov. Gretchen Whitmer called Biden a “great public servant” and emphasized her commitment to electing Democrats and stopping Donald Trump.

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.”

Future of the Democratic Nomination

Replacing Biden atop the Democratic ticket is likely to set off internal Democratic tremors as ambitious officials manoeuvre to become his successor. Factions have already formed around Harris, Whitmer, and Newsom. Harris, who broke a barrier as the first female vice president and enjoys strong support among African Americans, seems to be the heir apparent. However, her overall approval rating stood at only 32% in an NBC News poll released earlier this month.

Legal and Political Implications

The mechanics of putting a new name on ballots also give 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. 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.

Challenges for the Democratic Party

Biden’s departure is the latest in a series of jarring developments that have 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.

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.

Biden’s departure is likely to lead to a tumultuous period for the Democratic Party as they seek to unify around a new candidate and prepare for a challenging general election campaign against Trump.