In a development that sends the contest into uncharted territory, the US president announced that he will no longer be seeking a second term. He has thrown his support behind Kamala Harris, urging Democrats to donate to her campaign.

In a letter addressed to "my fellow Americans," Mr. Biden said it had been the "greatest honor of my life to serve as your president."

"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."

He has endorsed Mrs. Harris as the Democratic nominee, saying: "Democrats - it's time to come together and beat Trump."

Mr. Biden made the announcement in a statement posted to X, in which he said he would address the nation later this week to provide "details" about his decision.

"For now, let me express my deepest gratitude to all those who have worked so hard to see me re-elected. I want to thank Vice President Kamala Harris for being an extraordinary partner in all this work. And let me express my heartfelt appreciation to the American people for the faith and trust you have placed in me."

"I believe today what I always have: that there is nothing America can't do - when we do it together. We just have to remember we are the United States of America."

Mr. Biden's wife Jill Biden reposted the statement on X with a love heart.

Mr. Biden plans to serve out the remainder of his term in office, which ends at noon on January 20, 2025. But questions are already being asked about whether he should stand down from the presidency as well as withdrawing from the re-election race.

The Republican House of Representatives Speaker Mike Johnson has called on him to resign, saying, "If Joe Biden is not fit to run for president, he is not fit to serve as president.

What caused the sudden decision after long Speculation.

The seismic development comes after months of speculation about Mr. Biden's age and fitness to serve another four years - amid a series of high-profile gaffes and a disastrous debate performance.

Mr. Biden, 81, had previously insisted he was best placed to defeat Mr. Trump in the upcoming election but had faced calls from within his own party to step aside.

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

The decision caught many White House staff off guard, with several expressing shock that the president made the announcement while recovering from COVID at his Delaware home. He spoke with Mrs. Harris today ahead of the announcement, according to a source familiar with the conversation.

A source close to the president recently admitted "we're close to the end" after Barack Obama, the former president who picked Mr. Biden as his vice-president during his two terms in office, reportedly shared his concerns about his ally's re-election bid.