(CNN) US Senator and former Democratic presidential candidate Kamala Harris said Sunday she's endorsing Joe Biden for president, the latest in a series of high-profile announcements from Democrats backing the former vice president.

"When I started my run for president, I said America needs a president who reflects the decency and dignity of the American people; a president who speaks the truth; and a president who fights for those whose voices are too often overlooked or ignored. I still believe that to this day. That is why I am proud to announce I am endorsing my friend, Vice President Joe Biden, for President of the United States," Harris said in a statement on Sunday.

The California Democrat, who ended her 2020 presidential campaign in December 2019, also posted the announcement in a video on her Twitter account Sunday. She added that she would be in Detroit Monday to campaign with Biden. CNN previously reported that Harris was considering endorsing Biden.

Biden on Sunday morning thanked Harris for the endorsement, saying "from our family: thank you."

"Kamala — You've spent your whole career fighting for folks who've been written off and left behind — and no small part of that alongside Beau. From our family: thank you," the former Vice President said in a retweet of her endorsement.

The long-anticipated endorsement from Harris comes as a string of former 2020 presidential contenders have thrown their support behind Biden, including Minnesota Sen. Amy Klobuchar, former South Bend, Indiana, mayor Pete Buttigieg, and former Texas Rep. Beto O'Rourke.

And on Saturday afternoon at a rally in St. Louis, Missouri, it seemed like Biden may have gotten ahead of himself in announcing the news. As he was thanking all of his endorsers' supporters' for coalescing around him, he also mentioned Harris' name. "And by the way, to all of Amy's folks, to all of Pete's folks, to all of Kamala's folks, to all of the folks, to Beto's folks, I tell you what, what a gigantic difference it's made. We're going to unite this party and unite this country."

Harris came to her final decision to endorse Biden on Saturday, a source with knowledge of the situation told CNN. Biden and Harris have been in touch in recent days, but Harris officially decided to move forward with backing Biden after Massachusetts Sen. Elizabeth Warren dropped out of the race on Thursday, the source said. Harris waited to endorse because she did not want to go against Warren or Klobuchar, the source added.

Harris and Biden's past

Harris had a relationship with the Biden family before the 2020 presidential campaign. She served as California attorney general at the same time that Biden's late son, Beau Biden, was attorney general of Delaware. Joe Biden has publicly and privately encouraged Harris to stay involved in politics.

But during the first Democratic primary debate, the California senator confronted Biden on race issues, specifically about the time he spent in the Senate fighting against federally-mandated busing to desegregate schools and his comments on working with segregationist senators. While the viral moment helped Harris in the immediate post-debate polls, she later dropped significantly.

Once seen as a leading contender for the Democratic nomination, Harris ultimately ended her campaign in December after struggling for months to move her low poll numbers. She said in an announcement at the time that financial pressures had led her to end her bid.

The next day, Biden, when asked by reporters if he would consider Harris as a vice presidential pick said "of course."

"Of course I would," said Biden, turning around to answer the question as he walked onto his campaign bus. "Look, Senator Harris has the capacity to be anything she wants to be. I mean it sincerely. I talked to her yesterday. She's solid, she can be president someday herself, she can be vice president, she could go on to be a Supreme Court justice, she could be attorney general. I mean she has enormous capability."

This story has been updated with additional developments.