Biden, 81, endorsed Vice-President Kamala Harris to take over the ticket vacated by him, saying, "Today I want to offer my full support and endorsement for Kamala Harris to be the nominee of our party this year. Democrats -- it's time to come together and beat Trump."

The Democratic Party will need to nominate Harris formally at the convention in August, making her the first African-American woman and the first Indian-American to win the nomination of a major political party for the White House.

Harris's path from here to nomination is not automatic. It's not clear if she will have to face off rivals and how that process will unfold. Whether it will be settled based on the delegates won by the Biden-Harris ticket or voting at an open convention where the delegates will be free to consider and pick a rival candidate. Others who could seek the nomination include Governors Gavin Newsom of California, Gretchen Whitmer of Michigan, Andy Beshear of Kentucky, Josh Shapiro of Pennsylvania, Roy Cooper of North Carolina and Senator Mark Kelly, and entrepreneur and investor Mark Cuban.

Biden became the first incumbent President in 56 years to give up a chance to run again. It could not be immediately ascertained if he had also become the first US President to withdraw his re-election bid after winning the primaries and before the official coronation as the nominee at the party's national convention.

President Biden, who has been self-isolating at his home in Delaware with a Covid-19 infection, had been under mounting pressure to step aside and make way for a different candidate for the party, following a disastrous performance in the first presidential debate against his Republican rival, former President Trump.

"Over the past three-and-a-half years, we have made great progress as a Nation," he wrote in a letter addressed to "fellow Americans".

"Today, America has the strongest economy in the world. We've made historic investments in rebuilding our Nation, in lowering prescription drug costs for seniors, and in expanding affordable health care to a record number of Americans. We've provided critically needed care to a million veterans exposed to toxic substances. Passed the first gun safety law in 30 years."

"Appointed the first African-American woman to the Supreme Court. And passed the most significant climate legislation in the history of the world. America has never been better positioned to lead than we are today."

"I know none of this could have been done without you, the American people. Together, we overcame a once-in-a-century pandemic and the worst economic crisis since the Great Depression. We've protected and preserved our Democracy. And we've revitalised and strengthened our alliances around the world."

"It has been the greatest honour 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."

"I will speak to the Nation later this week in more detail about my 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."

Biden had merely thanked Harris for her cooperation in the first statement Biden had issued about his exit. He endorsed her fully in a subsequent statement.

"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."