President Joe Biden announced Sunday that he will end his re-election campaign, concluding his half-century political career and shaking up the presidential race less than four months before Election Day.

Biden, 81, couldn’t quell concerns within his party about his ability to serve and his chances against Donald Trump in November. He endorsed Vice President Kamala Harris to succeed him as the Democratic nominee.

“While I intended to seek re-election, it’s best for my party and country that I step down and focus on my duties for the rest of my term,” Biden wrote in a letter on X. “I will address the nation in more detail about my decision later this week.”

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

“My first decision as the 2020 nominee was picking Kamala Harris as Vice President, and it was the best one. Today, I fully support and endorse Kamala as our party’s nominee,” Biden posted.

Biden and Harris spoke multiple times Sunday before his announcement, according to a campaign source.

Harris thanked Biden for his “extraordinary leadership” and his “remarkable legacy.”

“I am honored by the President’s endorsement and intend to earn and win this nomination,” Harris said. “I will unite the Democratic Party and the nation to defeat Donald Trump and his extreme Project 2025 agenda.”

Biden, who had publicly insisted he would stay in the race, informed his senior team of his change of heart at 1:45 p.m. ET. His announcement was tweeted at 1:46 p.m.

His withdrawal caps a unique national political career. Biden mounted four presidential bids, served 36 years in the U.S. Senate, chaired powerful Judiciary and Foreign Relations committees, and was vice president for eight years under Barack Obama.

Biden’s exit from the race, less than a month before his party’s convention and a few months before the election, is unprecedented. The last sitting president to abandon a re-election bid was Lyndon Johnson in March 1968.

“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 is likely to cause internal Democratic strife as ambitious officials vie to succeed him. Harris, 59, seems to be the heir apparent, enjoying strong support among African Americans but holding only a 32% approval rating in a recent NBC News poll.