Harris, who dropped out of the race in December, announced her endorsement in a video posted to Twitter on Sunday.
“There is no one better prepared than Joe to steer our nation through these turbulent times, and restore truth, honor, and decency to the Oval Office,” she said in a statement. “He is kind and endlessly caring, and he truly listens to the American people.”
Harris will join Biden at his rally in Detroit on Monday, his campaign announced Sunday.
Harris is the latest former Democratic presidential candidate to throw their support behind Biden. Former South Bend, Indiana, Mayor Pete Buttigieg and Sen. Amy Klobuchar (D-Minn.) endorsed him after suspending their campaigns just ahead of Super Tuesday.
Former Texas Rep. Beto O’Rourke, who ended his bid for the White House in November, and former New York City Mayor Michael Bloomberg, who dropped out of the race last week, have also recently endorsed the former vice president.
Biden appeared to hint at Harris’ imminent endorsement during a campaign rally Saturday in St. Louis.
“To all of Amy’s folks, to all of Pete’s folks, to all of Kamala’s folks, to all of the folks who have, Beto’s folks, I’ll tell you what, what a gigantic difference it’s made,” he told the crowd. “We’re gonna unite this party and unite this country.”
After disappointing showings in the first few Democratic primary contests last month, Biden’s campaign got a major boost on Super Tuesday last week, winning 10 of the 14 states up for grabs. Sen. Bernie Sanders (I-Vt.) won the other four states, including the state with the most delegates up for grabs: California.
Harris was at times critical of Biden on the campaign trail. Her criticism during a Democratic debate in June of Biden’s record on racial segregation, including his previous remarks boasting about collaborating with two segregationist senators during his time in the Senate, marked the defining moment of the night.
“I do not believe you are a racist. And I agree with you when you commit yourself to the importance of finding common ground,” Harris had said. “But 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.”
She pointed out Biden’s previous opposition to busing programs that integrated schools and recalled her own experience as a member of the “second class to integrate her public schools” when she was a child.
But Harris struck a decidedly different tone in her endorsement on Sunday.
“We deserve a president with the knowledge and experience to reassure a nation weary of tweets, lies, and incompetence,” Harris said in her statement. “And we deserve a president who speaks to the best of who we are and challenges us to live up to our ideals.”
“I believe in Joe Biden,” she added, “and will do everything in my power to help elect him the next president of the United States.”