(AFP) - Joe Biden on Sunday dropped out of the US presidential election and endorsed Vice President Kamala Harris as the Democratic Party's new nominee, in a political earthquake that upends the already extraordinary race for the White House.

Biden, 81, said he was acting in the "best interest of my party and the country" by bowing to weeks of pressure after a disastrous June debate against Donald Trump stoked worries about his age and mental fitness.

The stunning move throws Democrats into fresh turmoil ahead of the November 5 election. But it could also reenergize the demoralized party, with Harris swiftly confirming her goal to become America's first woman president and to "defeat Donald Trump."

Trump reacted with a stream of posts on his Truth Social network, saying that because Biden is not "fit to run" for president, he is also not "fit to serve."

However, the dramatic shift will wrong-foot Republicans, whose campaign was solely focused on Biden and will now instead feature 78-year-old Trump -- the oldest presidential nominee in US history -- up against a far younger opponent.

The move also transforms what had been a highly unpopular and dragging Trump-Biden rematch into one of the most compelling presidential campaigns in modern American politics.

Biden's withdrawal had been widely expected at some point. The announcement finally came with no warning as he recovered from Covid at his Delaware beach house.

In a letter posted on X, Biden said it had been the "greatest honor of my life" to be president. He said he would address the nation later this week. The White House later said he had no public events scheduled for Monday.

"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 focus solely on fulfilling my duties as President for the remainder of my term," he wrote.

Shortly after, he offered his "full support and endorsement" for Harris, with his campaign filing official notice to change its name to "Harris for President."

Endorsements began streaming in for Harris almost immediately from Democratic big shots as well as those seen as potential rivals for the nomination, such as California Governor Gavin Newsom.

The Democratic fundraising group ActBlue meanwhile reported that Harris received $46.7 million in small-donor contributions in the hours just after announcing her campaign -- its "biggest fundraising day of the 2024 cycle."

- 'Unite our nation' -

Democrats must now scramble to confirm a new candidate at their party convention in Chicago on August 19.

Harris, the first Black and South Asian woman vice president in US history, praised Biden for his "selfless and patriotic act" and vowed to "earn and win" the nomination.

Still highly influential former Democratic president Barack Obama cautioned that "uncharted waters" lie ahead.

Biden's decision came after a period of enforced isolation, with only a few family members and aides around him to consult at his Rehoboth Beach home, as he nursed a Covid infection.

First Lady Jill Biden reacted by simply reposting his statement, along with two hearts.

In a clear sign of how Republicans will try to frame Harris's image, Trump's new running mate J.D. Vance underlined that she had been "every step of the way" with Biden, "the worst president in my lifetime."

- 'Mental decline' -

Biden's decision to exit caps a tense and chaotic period in the US election, with Trump having survived an assassination attempt at a campaign rally on July 13, and Democrats tearing themselves apart for weeks over whether Biden should quit.

The Democrat is the first president in US history to drop out so late in an election race.

Biden spent more than three weeks resisting calls to step down following the shock of the June 27 debate, during which he often lost his train of thought and stood with mouth agape.

Harris meanwhile struggled to make an impact in her first years in the White House, but performed strongly on the campaign trail on key issues such as abortion.

In recent weeks, the Biden campaign has reportedly been quietly carrying out a head-to-head survey of voters measuring how the former California prosecutor matched up against convicted felon Trump.

Biden took office in January 2021 pledging to heal the "soul of America" after four turbulent years under Trump and the shock of the January 6, 2021 Capitol assault by his supporters.

Overcoming a reputation for verbal flubs, Obama's former vice president gave strong backing to Ukraine's battle against Russia's 2022 invasion, pushed through a massive Covid recovery plan and historic green industry subsidies.

But he faced criticism over the catastrophic US withdrawal from Afghanistan, high inflation, and his support for Israel's war in Gaza -- while concerns over his age only mounted.

© Agence France-Presse