US President Joe Biden ended his re-election campaign on Sunday after fellow Democrats lost faith in his mental acuity and ability to beat Donald Trump, leaving the presidential race in uncharted territory.

Biden, in a post on X, said he will remain in his role as president and commander-in-chief until his term ends in January 2025 and will address the nation this week.

“It has been the greatest honour of my life to serve as your President. And 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.

By dropping his re-election bid, he clears the way for Vice President Kamala Harris to run at the top of the ticket, the first Black woman to do so in the country’s history.

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Later, in a separate post, Biden nominated Harris as the nominee of the Democrat party this year.

“My fellow Democrats, I have decided not to accept the nomination and to focus all my energies on my duties as president for the remainder of my term,” he said.

“My 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,” he added.

“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. Let’s do this,” Biden said.

Biden as of Saturday night had planned to stay in the 2024 presidential race but told senior staff on Sunday afternoon that he was withdrawing, a source familiar with the matter said.

“Last night the message was [to] proceed with everything, full speed ahead,” the source told Reuters, speaking on condition of anonymity.

“At around 1:45pm today: the president told his senior team that he had changed his mind.”

It was unclear whether other senior Democrats would challenge Harris for the party’s nomination, who was widely seen as the pick for many party officials — or whether the party itself would choose to open the field for nominations.

Biden’s announcement follows a wave of public and private pressure from Democratic lawmakers and party officials to quit the race after his shockingly poor performance in a televised debate last month against Republican rival Donald Trump.

Biden’s failure at times to complete clear sentences took the public spotlight away from Trump’s performance, in which he made a string of false statements, and trained it instead on questions surrounding Biden’s fitness for another 4-year term.

Days later he raised fresh concerns in an interview, shrugging off Democrats’ worries and a widening gap in opinion polls, and saying he would be fine losing to Trump if he knew he’d “gave it my all.”

His gaffes at a Nato summit — invoking Russian President Vladimir Putin’s name when he meant Ukrainian President Volodymyr Zelenskiy and calling Harris “Vice President Trump” — further stoked anxieties.

Only four days before Sunday’s announcement, Biden was diagnosed with Covid-19 for a third time, forcing him to cut short a campaign trip to Las Vegas. More than one in 10 congressional Democrats had called publicly for him to quit the race.

Biden’s historic move — the first sitting president to give up his party’s nomination for re-election since President Lyndon Johnson in March 1968 — leaves his replacement with less than four months to wage a campaign.

‘Crooked Joe not fit to run’

Republican nominee for US election race Donald Trump said, “Crooked Joe Biden was not fit to run for President, and is certainly not fit to serve — and never was!”

“He only attained the position of president by lies, fake news, and not leaving his basement,” Trump said on his Truth Social platform.

“All those around him, including his doctor and the media, knew that he wasn’t capable of being president, and he wasn’t,” he said.

“And now, look what he’s done to our country, with millions of people coming across our border, totally unchecked and unvetted, many from prisons, mental institutions, and record numbers of terrorists,” he added.

“We will suffer greatly because of his presidency, but we will remedy the damage he has done very quickly,” he said.

“Make America great again!” Trump said.

How would a nominee be chosen?

There would likely be a free-for-all of sorts between the Democratic heavyweights vying for the job.

Candidates would have to get signatures from 600 convention delegates to be nominated. There are expected to be some 4,672 delegates in 2024, including 3,933 pledged delegates and 739 automatic or superdelegates, according to Ballotpedia.

If no one gets a majority of the delegates, then there would be a “brokered convention” in which the delegates act as free agents and negotiate with the party leadership to come up with a nominee.

Rules would be established and there would be roll call votes for the names placed into nomination.

It could take several rounds of voting for someone to get a majority and become the nominee. The last brokered convention when Democrats failed to nominate a candidate on the first ballot was in 1952.