US President Joe Biden, after nearly a month of mounting pressure from his own party, dropped his reelection bid on Sunday and endorsed Vice-President Kamala Harris to head the Democratic ticket in November.

"It has been the greatest honor of my life to serve as your president," he wrote in a letter posted to social media. "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 focus entirely on fulfilling my duties as president for the remainder of my term."

Biden's endorsement for Harris came in a subsequent post on X.

"Today I want to offer my full support and endorsement for Kamala to be the nominee of our party this year," Biden said in a post to X on Sunday. "Democrats — it's time to come together and beat Trump."

"I will do everything in my power to unite the Democratic Party — and our nation — to defeat Donald Trump," said Harris, adding that she was honored to have Biden's endorsement and wrote on X that "my intention is to earn and win this nomination".

The Democratic candidate, who will be nominated at the Democratic National Convention, which starts on Aug 19 in Chicago, will face Trump, 78, the Republican nominee and former president.

Biden, 81, whose term ends on Jan 20, 2025, said he will address the nation about his decision later this week. He is the first sitting US president to not seek reelection since Lyndon B. Johnson in March 1968.

"Should she win the nomination, the nation will watch a multiracial woman compete in a presidential general election for the first time in history," Debbie Walsh, director at the Center for American Women & Politics, said of Harris, 59, who is of Jamaican and Asian Indian descent.

Harris previously served as a US senator from California and as California attorney general.

She immediately got to work shoring up her support on Sunday, and as the evening wore on she appeared to be quickly building momentum. The North Carolina and South Carolina delegates pledged their support to her on Sunday evening as did a growing list of Democratic governors and members of Congress.

While Biden did endorse Harris, her nomination is not guaranteed, and there could be a rare open convention, also referred to as a "floor fight". There also had been talk of Democrats selecting a nominee virtually before the convention.

But Representative Nanette Barragán of California, the chair of the Congressional Hispanic Caucus, who said she spoke with Harris on Sunday, said the vice-president wants to forego a virtual roll call for the nomination process and instead hold a process that adheres to regular order.

Harris needs the backing of 1,969 of the 3,936 Democratic delegates to secure her nomination, Reuters reported.

"Biden does not control his delegates: He cannot tell them whom to vote for next month. And party politics being what it is, they are most likely to go with whomever they think has the best chance to defeat Mr. Trump," The New York Times reported on Sunday.

Among the names mentioned as other potential Democratic presidential candidates are governors Gavin Newsom of California, Gretchen Whitmer of Michigan, Josh Shapiro of Pennsylvania, J.B. Pritzker of Illinois and Andy Beshear of Kentucky.

Although Newsom wrote on X on Sunday: "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, @KamalaHarris."

Whitmer and Beshear issued statements praising Biden but did not mention Harris.

Biden and Harris together had raised a massive campaign war chest of $240 million through various entities as of June 30, according to the Times.

ActBlue, a Democratic fundraising platform, announced it had collected $46.7 million as of 9 pm ET from small-dollar donations for Harris' campaign.

Media reports circulated last week that Biden would step down on Sunday, but the stories lost steam as Biden angrily dismissed them.

Biden's political fortunes began to crumble after his dismal performance in a debate with Trump on June 27. Gaffes in subsequent interviews and appearances led to more doubts about whether he should continue.

Former president Barack Obama praised Biden, his former vice-president, in a statement on Medium.com on Sunday, saying Biden "wouldn't make this decision unless he believed it was right for America".

Obama said Biden "pointed us away from the four years of chaos, falsehood, and division that had characterized Donald Trump's administration".

"We will be navigating uncharted waters in the days ahead. But I have extraordinary confidence that the leaders of our party will be able to create a process from which an outstanding nominee emerges."

Independent candidate Robert F. Kennedy Jr, in comments from his family compound in Hyannis Port, Massachusetts, said neither party will address what he considers the major issues facing the US.

Agencies contributed to this story.