Washington, 22 July 2024 (dpa/MIA) — US President Joe Biden announced on Sunday that he is withdrawing from the race to be the next president, in a historic decision that throws the Democratic Party into uncharted territory as Republican rival Donald Trump's campaign gains steam.

Biden said that Vice President Kamala Harris had his "full support" to replace him on the ballot in November's election. He also said that he will finish out the remainder of his term, which ends on January 20, 2025.

"It has been the greatest honor 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 in a letter.

But with the party's presumptive nominee now gone and just over 100 days to go, the clock is ticking for Democrats to begin the fraught process of replacing him and launching a viable campaign to take on Trump.

Harris vows to 'earn and win' nomination

Hours after Biden's bombshell withdrawal, Harris issued a statement in which she promised to do "everything in my power" to defeat Trump.

"I am honored to have the president’s endorsement and my intention is to earn and win this nomination,” she said.

In endorsing Harris, Biden wrote: "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."

Harris, 59, is the first female vice president in US history, as well as the first black woman and person of South Asian descent. Prior to becoming Biden's deputy, she was a senator from California and the state's attorney general.

California Governor Gavin Newsom, a rising star in the party who has long been seen as having White House ambitions, praised Harris.

"Tough. Fearless. Tenacious," he wrote on social media platform X.

"With our democracy at stake and our future on the line, no one is better to prosecute the case against Donald Trump's dark vision and guide our country in a healthier direction than America’s Vice President, Kamala Harris."

It was not immediately clear if other candidates would scramble to put themselves forward to be officially anointed Biden's replacement at the national party convention next month in Chicago.

Former Democratic president Bill Clinton and former secretary of state Hillary Clinton jointly said they would like to have Harris run.

The Congressional Black Caucus, a key group of Biden allies in Congress, also threw their support to Harris.

Former president Barack Obama described Biden as "patriot of the highest order" but did not say who should replace him.

Leaders praise Biden's decision and work

Biden's decision sent shockwaves across the country - and globe - and came after massive pressure from within his own party not to put himself forward as the official nominee.

Biden's decision deserved respect, according to German Chancellor Olaf Scholz.

"My friend [US President] Joe Biden has achieved a lot: for his country, for Europe, for the world," Scholz wrote on X.

"Thanks to him, transatlantic cooperation is close, NATO is strong and the USA is a good and reliable partner for us. His decision not to run again deserves respect," Scholz wrote.

Ukrainian President Volodymyr Zelensky said he respected Biden's "tough but strong decision" not to run again.

Zelensky was grateful to Biden for his "unwavering support for Ukraine's fight for freedom," he wrote.

US Defense Secretary Lloyd Austin has paid tribute to Biden as an "extraordinary guardian" of the country's security.

"President Biden has been a stalwart defender of free government, a fearless foe of tyranny, and a historic champion of an open world of rules, rights, and responsibilities."

Biden's age and debate struggles fuel Trump's attacks

A television debate performance against Trump at the end of June, during which 81-year-old Biden struggled several times to give coherent answers or challenge falsehoods uttered by Trump, prompted nationwide concern about whether he was still mentally fit for office.

The issue of Biden's age has since dominated the US election campaign, overshadowed a NATO summit, and been used repeatedly by Trump, 78, to attack his opponent as old and feeble.

"Crooked Joe Biden was not fit to run for President, and is certainly not fit to serve - And never was!" wrote former president Trump on his online platform Truth Social in response to Biden's announcement.

Opinion polls at both the national level and in the key battleground states that will determine this year's winner of the presidency have shown Trump pulling ahead by considerable margins.

The attempted assassination of Trump at a rally in Pennsylvania on July 13 reinforced a sense of unity among Republicans, with the party's fervour for their candidate on full display at last week's national convention in Milwaukee.