The 78-year-old Vermont senator’s announcement, made in a conference call with staff on Wednesday.
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.
“The path toward victory is virtually impossible,” Sanders told supporters as he congratulated Biden, while thanking an “unprecedented grassroots political campaign” that relied entirely on small donations from millions of his backers.
He spoke of a “difficult and painful” decision, in his video message.
“The former vice president is a very decent man whom I will work with to move our progressive ideas forward”, he added.
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, a democratic socialist whose progressive agenda pulled the party sharply to the left, also mounted an unexpectedly strong challenge in 2016 to eventual Democratic nominee Hillary Clinton.
After shooting to an early lead in the race, he faded quickly after losing South Carolina in late February and has been under pressure to halt his campaign after Biden won resounding victories in primary contests on March 17 in Florida, Arizona and Illinois.
The departure of Sanders, the last remaining rival to Biden, sets up a battle between the 77-year-old former vice president and Republican President Trump, 73, who is seeking a second four-year term in office.
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