United States 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 on social media. "And 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 entirely on fulfilling my duties as president for the remainder of my term."

Biden's endorsement of Harris came in a subsequent post on social media platform X.

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

Harris said, "I will do everything in my power to unite the Democratic Party- and our nation — to defeat Donald Trump." She added 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, to be nominated at the Democratic National Convention, which starts on Aug 19 in Chicago, will face former president Trump, 78, the Republican nominee.

While Trump said on Sunday that Harris would be easier to defeat than Biden, he continued to criticize Biden, telling NBC News, "There has never been a president who has done such damage to our country, from energy independence to letting in millions and millions of illegal immigrants."

House Speaker Mike Johnson, a Republican, wrote on social media: "The Democrat Party forced the Democrat nominee off the ballot, just over 100 days before the election. Having invalidated the votes of more than 14 million Americans who selected Joe Biden to be the Democrat nominee for president, the self-proclaimed 'party of democracy' has proven exactly the opposite."

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

Harris began shoring up support on Sunday. The North Carolina and South Carolina delegates pledged their backing for her on Sunday evening, as did some governors and members of Congress.

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

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

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. Some of them also could be considered as potential vice-presidential running mates for Harris.

Biden and Harris together had raised a $240 million war chest as of June 30, according to The New York Times.

Media reports circulated last week that Biden, 81, would step down on Sunday, but the stories lost steam after Biden angrily dismissed them.

The president's political fortunes began to crumble after his dismal performance in a debate with Trump on June 27.

Former president Barack Obama praised Biden, his former vice-president, in a statement on Medium.com on Sunday, saying that 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".

Agencies contributed to this story.