On Sunday, the incumbent said he would not run for a second term, with his party now scrambling to find a replacement to take on Republican Donald Trump in the November election.
Here’s how we got here and how things can go from here.
Why did Joe Biden quit?
Mr Biden said it was “in the best interest of the party and the country that I step down”.
He added that he would speak in more detail this week to detail his decision.
The president’s aides have previously said that only his health will prevent him from running in November — which could suggest that the diagnosis played a role.
However, Mr. Biden has been under pressure for some time.
And while he has long been gaffe-prone, his performances have become increasingly erratic in recent weeks.
This has reportedly prompted senior Democrats including Barack Obama and Nancy Pelosi to call for Mr Biden to resign.
Joe Biden will be president once again
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How will it be chosen?
Democratic National Committee Chairman Jaime Harrison said there would be no automatic coronation to replace Mr Biden.
He added: “This process will be governed by the rules and procedures of the party. Our delegation is ready to take responsibility as soon as possible to send the candidate to the American people.
This is an unprecedented situation and the Democratic Party has not – as of Monday morning – offered many details about what happens next.
Any potential candidate would have to secure the nomination of about 4,700 Democratic convention delegates.
This includes those who have pledged to Mr. Biden, as well as elected officials, former presidents, and other party elders known as superdelegates.
There has been some confusion over whether Kamala Harris, the current vice president, will automatically be pushed forward for the top job. While he was the obvious candidate, the Democratic rules only state that delegates “in good conscience” vote for the candidate they choose to represent them, with no defector mechanism. And some in the party have endorsed an open nomination process.
The person, who spoke on condition of anonymity to discuss internal conversations, said Ms Harris would benefit from a competitive mini-primary ahead of the Democratic convention, as it would help seal her bona fides as a strong candidate and dampen criticism. he has been undemocratically anointed.
It’s a charge Trump’s campaign has used against him, and it could loom large in the battle for undecided voters in embattled states.
When will his replacement be announced?
Democrats want Ms. Harris to be on the ballot has been given a vote of confidence but do not want a crowded summer for competing candidacies.
The party will officially choose its presidential candidate at its convention on August 19 and will want to campaign on that message.
Vice President Kamala Harris
AP
Minutes after announcing his resignation, Mr. Biden gave his “full support” to Harris, the party’s favorite for the nomination, to become the Democratic nominee.
Harris also thanked Mr Biden for his “extraordinary leadership” and for performing a “selfless and patriotic act” by stepping aside.
He said: “My first decision as the party’s nominee in 2020 was to pick Kamala Harris for Vice President. And it was the best decision I ever made.
“Today I want to offer my full support and endorsement for Kamala to be our party’s nominee this year. Democrats – it’s time to come together and defeat Trump. Let’s work on this.”
Hollywood stars and donors have also supported him.
Ms Harris could become the first female president of the US and would be an obvious choice to support the continuity of the party.
Gavin Newsom is unlikely to challenge Ms Harris at this late stage
AP
As of now, everyone else on the list may and has not been confirmed to try to run but is in a good position within the Democratic Party.
California governor Mr. Newsom will end his second and final term in office and is said to be looking for the top job.
The Pennsylvania governor is a rising star but his time may come in 2028.
The California representative has been mentioned as another potential name in the frame.
But he won’t be open this time, he said, tweeting: “I’m proud to endorse @KamalaHarris as our nominee.
“Her candidacy as the first African-American woman and the first Asian American will be an energy in our party. Our party can now reveal a message of hope and vision for the future.
