US Senator Bernie Sanders has suspended his presidential campaign, making former vice president Joe Biden the presumptive nominee to face President Donald Trump in November's election.
"The path toward victory is virtually impossible," Mr Sanders said in a live speech streamed to supporters.
"I have concluded that this battle for the Democratic nomination will not be successful. And so today, I am announcing the suspension of my campaign."
The 78-year-old, from Vermont, a democratic socialist whose progressive agenda pulled the party sharply to the left, shot into an early lead in the Democratic race.
But he faded quickly after losing South Carolina in late February as moderate Democrats consolidated their support behind Mr Biden.
The departure of Mr Sanders, the last remaining rival to Mr Biden, sets up a race between the 77-year-old former vice president and Mr Trump, 73, who is seeking a second four-year term in office.
The decision comes as the US grapples with a coronavirus outbreak that upended the nominating schedule, with some primaries postponed and others up in the air.
Mr Sanders, who also mounted an unexpectedly strong challenge in 2016 to eventual Democratic nominee Hillary Clinton, had been under pressure to halt his campaign after Mr Biden won resounding victories in primary contests on 17 March in Florida, Arizona and Illinois.
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Some allies had encouraged Mr Sanders to stay in the race to further influence Mr Biden's policy positions.
But the coronavirus crisis shifted public focus away from the campaign and, with all rallies cancelled, Mr Sanders had little opportunity to get his message across.
Mr Biden tweeted his thanks to Mr Sanders, hailing him as "a good man, a great leader, and one of the most powerful voices for change in our country."
President Trump, who has courted Mr Sanders supporters and said the senator was treated unfairly by the Democratic Party, reacted quickly on Twitter.
"This ended just like the Democrats & the DNC wanted, same as the Crooked Hillary fiasco. The Bernie people should come to the Republican Party, TRADE!," Mr Trump wrote.
Bernie Sanders is OUT! Thank you to Elizabeth Warren. If not for her, Bernie would have won almost every state on Super Tuesday! This ended just like the Democrats & the DNC wanted, same as the Crooked Hillary fiasco. The Bernie people should come to the Republican Party, TRADE!— Donald J. Trump (@realDonaldTrump) April 8, 2020
Many of Mr Sanders' policy positions have become part of the mainstream Democratic Party debate, including his Medicare for All proposal that would create a government-run healthcare system to replace the current blend of private medical insurance and public programmes.
He also advocated a $15 minimum wage, tuition-free public colleges and higher taxes on the wealthy.
"Our movement has won the ideological struggle in so-called red states, blue states, and purple states," Mr Sanders said.