US President Joe Biden has told Americans in a televised address that he decided to end his faltering re-election campaign in a bid to save US democracy.
Mr Biden, 81, said he felt his record as president "merited a second term" but that "nothing can come in the way of saving our democracy".
He said he endorsed US Vice-President Kamala Harris to unite their fellow Democrats and the country.
The White House speech marked his first public appearance since he left the race on 21 July, paving the way for Ms Harris to run for the party's nomination.
Pressure had been building on him from colleagues and donors to step aside over doubts about his ability to serve another term following a disastrous debate with Republican Donald Trump.
It was notable that he did not address how his campaign unravelled and the fact that defeat to Mr Trump looked more likely the longer he resisted calls for him to quit.
He also did not mention the questions raised about his frailty and mental competence, issues which have dominated the race for months.
Opinion polls so far suggest a Ms Harris bounce since she took over Mr Biden's mantle, making it a tight race between her and the former president if she wins the nomination, which looks likely.
The president began his address with the words: "I revere this office, but I love my country more."
He spoke for 11 minutes from the Oval Office, with his wife, Jill, son, Hunter, and daughter, Ashley, in the room alongside some key White House staff.
“I have decided the best way forward is to pass the torch to a new generation," added Mr Biden, who has been in public office for over half a century. "That is the best way to unite our nation."
The president vowed to "finish the job for the American people" before the end of his term in January next year and called Ms Harris, 59, an "experienced, tough, capable" partner.
Mr Biden said he believed his record warranted a second term but personal ambition should not "come in the way of saving our democracy".
"The great thing about America is here, kings and dictators do not rule. The people do. History is in your hands. The power is in your hands. The idea of America lies in your hands."
As they were being ushered out of the room, the White House press corps heard applause from family members and staff as the president spoke softly.
Trump, 78, posted on his social media platform, Truth Social, on Wednesday night that the president “was barely understandable, and sooo bad!"
Speaking earlier in the day at a rally in Charlotte, North Carolina, the Republican attacked both Ms Harris and Mr Biden, whom he called the most damaging president in US history.
"For three-and-a-half years lying Kamala Harris has been the ultraliberal driving force behind every single Biden catastrophe," he said.
“She is a radical left lunatic who will destroy our country if she ever gets the chance to get into office."
At a White House news briefing earlier in the day, press secretary Karine Jean-Pierre said Mr Biden's exit was "not about his health", but declined to give further details.
She also denied to reporters there had been any "cover-up" over Mr Biden's cognitive abilities.
Ms Harris, for her part, is hitting the campaign trail.
Earlier on Wednesday, she addressed the historically black Zeta Phi Beta sorority in Indianapolis, telling members that her new campaign is "not playing around" and asked for their help to get elected.
“In this moment, I believe we face a choice between two different visions for our nation, one focused on the future, the other focused on the past,” she said. “And with your support, I am fighting for our nation’s future.”
She would be the first black, South-Asian American and woman president, though she needs black voters to turn out in November.
However, Black Lives Matter, the activist group, said in a statement on Tuesday that Democrats were seeking to "manipulate Black voters" by installing Ms Harris at the behest of billionaire donors in a "blatant disregard for democratic principles".
Mr Biden is the first sitting president not to seek re-election since 1968 when Lyndon Johnson halted his campaign as his popularity sank during the Vietnam War.
Some Republicans have called for Mr Biden to step down now as president, arguing that he cannot run the country if he cannot run for office - a charge the White House has rejected.
On Thursday, Mr Biden is expected to meet Israeli Prime Minister Benjamin Netanyahu at the White House, as well as with the families of US hostages still being held in Gaza.
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