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.
Mr Biden, 77, regained his lead in the race following the Super Tuesday Democratic primaries last week, winning 10 of the 14 states that voted.
The list of possible candidates who might realistically win the nomination then narrowed to become a contest between centrist Mr Biden and the 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.”
“We need a leader who really does care about the people, and can therefore unify the people,” the senator added.
The two have, however, previously clashed during the presidential debates.
Last June, Ms Harris attacked Mr Biden over his previous opposition to a policy combating segregation in schools.
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.
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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.