Bernie Sanders, the left-wing senator, is dropping out of the race to be the Democratic Party's nominee for US president, leaving former vice president Joe Biden as the presumptive candidate to run against incumbent Donald Trump in November.
"The path towards victory is virtually impossible," Sanders said, thanking an "unprecedented grassroots political campaign" that relied entirely on small donations from millions of supporters.
He spoke of a "difficult and painful" decision, in a video message.
Sanders called Biden a "decent man" and vowed to "stand together" with the former vice president to defeat Trump later this year.
The move comes as Biden built a clear delegate lead over Sanders, even as the primary season has been throw into disarray by the coronavirus outbreak.
One key issue to be watched is how Biden will now try to woo the left wing of the party, with a small but vocal segment of Sanders' supporters sceptical of the former vice president, who is seen as an establishment figure.
In a tight race against Trump, Biden could end up needing most of Sanders' energetic movement.
In a statement, Biden heaped praise on Sanders for drawing attention to injustices and inequalities and for creating a movement based on ideas. "Senator Sanders and his supporters have changed the dialogue in America," Biden said.
He also vowed to listen to Sanders' supporters, and urged them to support his campaign. "I hope you will join us. You are more than welcome. You're needed," Biden said.
Sanders had repeatedly vowed to support any Democrat against Trump, whom he often refers to as the "most dangerous president in American history."
Trump weighed in immediately on Twitter, and tried to pull Sanders' supporters to his side. "The Bernie people should come to the Republican Party," Trump wrote.
During a press conference, Trump said he and Sanders share similar views on global trade.