California Senator Kamala Harris has endorsed Joe Biden with "great enthusiasm" as the Democratic party's US presidential candidate.
"Biden has served our country with dignity and we need him now more than ever," she said in a Twitter post.
Her announcement is another boost for Mr Biden, the Democratic front runner to take on Donald Trump in November.
Ms Harris, seen as a rising star within the party, dropped out of the presidential race in December.
Support for Mr Biden surged in the Super Tuesday Democratic primaries last week, with the 77-year-old winning 10 of the 14 states that voted.
The race to the Democratic nomination has in effect become a contest between Mr Biden, a centrist, and left-wing Senator Bernie Sanders.
Throwing her support behind Mr Biden on Sunday, Ms Harris, 55, said in a recorded message on Twitter: "I really believe in him and I have known him for a long time."
.@JoeBiden has served our country with dignity and we need him now more than ever. I will do everything in my power to help elect him the next President of the United States. pic.twitter.com/DbB2fGWpaa— Kamala Harris (@KamalaHarris) March 8, 2020
"We need a leader who really does care about the people, and can therefore unify the people," the senator added.
Mr Biden later thanked Ms Harris for her support. "Kamala - You've spent your whole career fighting for folks who've been written off and left behind… from our family: thank you," he tweeted.
The two have, however, previously clashed during the presidential debates.
Ms Harris - the only black woman in the Democratic field - pilloried Mr Biden for having recently reminisced about working with two Democratic senators who favoured racial segregation.
She said at the time that she did not believe he was a racist, but added: "It was hurtful to hear you talk about the reputations of two United States senators who built their reputations and career on the segregation of race in this country."
Mr Biden's wife, Jill, said the senator's comments were "the biggest surprise" of the campaign, adding: "The one thing you cannot say about Joe is that he's a racist... I mean, he got into politics because of his commitment to civil rights."
Ms Harris, a fierce critic of Mr Trump, said she would be campaigning with Mr Biden in Detroit on Monday.
Separately, US civil rights activist Jesse Jackson on Sunday endorsed Mr Sanders, saying the Vermont senator's progressive social and economic policies would give black Americans "the best chance to catch up".