REHOBOTH BEACH, United States: US President Joe Biden announced Sunday that he was dropping out of his reelection battle with Donald Trump, in a historic move that plunges the already turbulent 2024 White House race into uncharted territory.

"It has been the greatest honor of my life to serve you as president," the 81-year-old Democrat said in a letter in X, posted while he was recovering from Covid at his beach house in Delaware.

"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."

Biden said he would "speak to the nation later this week in more detail about my decision."

The Democratic Party is now plunged into chaos and needs to find a new candidate by November's election, with Vice President Kamala Harris the frontrunner.

Biden bowed out after weeks of pressure that began with a disastrous debate performance that raised fears over his health.

The stunning move makes Biden the first president in US history to pull out so late in an election race, and the first to bow out because of concerns over his mental acuity and health.

Biden spent more than three weeks resisting calls to step down following the shock of the June 27 debate, at one point insisting that only the "Lord Almighty" could convince him to back out.

In a bid to show he was up to the job, he gave a number of interviews and what was billed as a "big boy" press conference in which he took numerous questions, but made further gaffes including calling Harris "Vice President Trump."

A tide of voices within his own party calling on him to go, starting with donor and actor George Clooney and ending with former president Barack Obama, sealed his fate.

The end finally came shortly after Biden had been diagnosed with Covid, forcing him off the campaign trail and into isolation.

Biden's decision to pull out also caps a tense and chaotic period in the US election, with Trump having survived an assassination attempt at a campaign rally on July 13.

He joins a small club of US presidents who have decided to throw in the towel after just one term, with the last being Lyndon Johnson in 1968 - a year also marked by political turmoil and violence.

Johnson's replacement as nominee, then-vice president Hubert Humphrey, went on to lose heavily to Richard Nixon.

But Democrats are counting on Harris to fare better, and hoping that she can prevent convicted felon Trump from making a sensational comeback to the Oval Office.

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

While Harris struggled to make an impact in her first years in the White House, she has emerged in the last year as a strong performer on the campaign trail on key messages such as abortion rights.

The former prosecutor has also made much of her life story as the first woman in US history to hold the vice presidency, as well as the first person of Black and South Asian origin.

Harris is now set to be nominated at the Democratic National Convention in Chicago on August 19 in what promises to be a dramatic moment - and a heartrending one for Biden.

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, Barack Obama's former vice president pushed through a massive Covid recovery plan and a green industry scheme.

US allies welcomed his pledge that "America is back" following Trump's trampling on international alliances, and his strong support for Ukraine as it battled Russia's 2022 invasion.

But he faced criticism over the catastrophic US withdrawal from Afghanistan and inflation that meant overstretched Americans ignored otherwise positive economic numbers.

Behind it all were the ongoing concerns about his age with a series of senior moments, including tripping up the stairs to Air Force One and falling off his bike, contributing to the doddery image played up by Republicans.

  - AFP