President Joe Biden has announced he is standing down as the Democrat's nominee for the upcoming Presidential election.

In a statement posted on X, the President said he would address the nation later today and endorsed his Vice President Kamala Harris as his choice to be the Democrat nominee.

It comes after fellow Democrats lost faith in his mental acuity and ability to beat Donald Trump, leaving the presidential race in uncharted territory.

The president said "it is in the best interest of my party" to stand down and "focus solely on fulfilling my duties as President for the remainder of my term".

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.

In a statement he wrote: "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."

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. Biden, 81, did not mention her when he announced his move.

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.

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Before his announcement, 37 Capitol Hill Democrats had called on the president to step aside.

A source close to Biden told Reuters that, as of Saturday night the President had planned to continue 2024 run but told senior staff at 1.45pm EDT (6.45pm GMT) on Sunday that he was dropping out.

In a post on social media, President Biden 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.

"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."

Earlier this week, Biden tested positive for Covid-19 while on a trip to Las Vegas on Wednesday and is self-isolating after experiencing mild symptoms.

White House press secretary Karine Jean-Pierre announced the positive test for the 81-year-old Democrat after Biden cancelled a speech due to the diagnosis.

Naomi Biden, granddaughter of President Joe Biden, said she is “nothing but proud today” of her grandfather. She wrote on X: "I’m nothing but proud today of my Pop, our President, Joe Biden, who has served our country with every bit of his soul and with unmatched distinction.

"Not only has he been—and will continue to be—the most effective president of our lifetime, but he has likely already cemented himself as the most effective and impactful public servant in our nation’s history."

Former President Donald Trump told CNN that Biden would go "down as the single worst president by far in the history of our country."

While it’s unclear who the Democratic nominee will be, Trump said he thinks Vice President Kamala Harris will be easier to defeat than Biden would have been.

Meanwhile, Senate Majority Leader Chuck Schumer praised President Joe Biden for putting "his party, and our future first."

He said: "Joe Biden has not only been a great president and a great legislative leader but he is a truly amazing human being. His decision of course was not easy, but he once again put his country, his party, and our future first...Joe, today shows you are a true patriot and great American."

During the primary race, Biden accumulated more than 3,600 delegates to the Democratic National Convention to be held in Chicago in August. That was almost double the 1,976 needed to win the party's nomination.

Unless the Democratic Party changes the rules, delegates pledged to Biden would enter the convention “uncommitted," leaving them to vote on his successor.

Democrats also have a system of “superdelegates," unpledged senior party officials and elected leaders whose support is limited on the first ballot but who could play a decisive role in subsequent rounds.

Biden beat Trump in 2020 by winning in the key battleground states, including tight races in Pennsylvania and Georgia. At a national level, he bested Trump by more than seven million votes, capturing 51.3% of the popular vote to Trump's 46.8%.