Sen. Kamala Harris (D-CA) endorsed former Vice President Joe Biden on Sunday, joining a growing group of prominent politicians who have backed Biden ahead of key races on March 10 and 17.

“I believe in Joe. I really believe in him and I have known him for a long time,” Harris said. “One of the things that we need right now, is we need a leader who really does care about the people and who can therefore unify the people. And I believe Joe can do that.”

.@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

Biden thanked Harris in a tweet, even bringing up his late son, who Biden said worked closely with the senator when she was attorney general in California and while he held the same position in Delaware.

“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,” Biden wrote.

The endorsement was made just five days after Biden celebrated a sweeping win on Super Tuesday, taking the lead in the delegate count. And it is a timely one, as Biden hopes to build upon that lead in the next major day of contests Tuesday, when Democrats in Idaho, Michigan, Mississippi, Missouri, North Dakota, Washington, and abroad will get to select their choice for the party’s nominee.

While the day does not have as many delegates on offer as Super Tuesday’s 1,344, a significant number — 365 — will be awarded; enough for Biden to pick up a commanding delegate lead, or for Sen. Bernie Sanders to bridge the 79-delegate gap that currently separates the two frontrunners.

Despite being one of the top five candidates in the polls early in the 2020 Democratic primary, Harris dropped out of the race in December amid declining poll numbers and difficulties with fundraising. Before her exit, however, she had a viral moment. At the first Democratic presidential debate, Harris had a tense exchange with Biden on the issue of school busing, pointing out that a bill he sponsored in 1975, which would have restricted the use of federal funds for busing meant to desegregate schools, could have kept her from attending her elementary school.

“You also worked with [segregationist senators] to oppose busing,” Harris said on the debate stage. “And there was a little girl in California who was part of the second class to integrate her public schools and she was bused to school every day. And that little girl was me.”

That history makes Harris’s endorsement even more notable, and adds credence to the narrative that it is Biden who can best unify the Democratic Party in its effort to defeat President Donald Trump.

Biden has a growing list of powerful Democrats backing him

With her endorsement, Harris joins a growing group of former 2020 candidates who have endorsed Biden, a list that includes Sen. Amy Klobuchar, former South Bend, Indiana, Mayor Pete Buttigieg, former Texas Rep. Beto O’Rourke, and former Massachusetts Gov. Deval Patrick.

And Biden’s formidable group of supporters also includes a number of other key lawmakers on the local, state, and federal levels who have endorsed him in recent days. In fact, dozens have endorsed Biden in the last 48 hours, including:

Rep. Sheila Jackson Lee (D-TX)

Atlanta Mayor Keisha Lance Bottoms

Former Sen. Heidi Heitkamp (D-ND)

Former Missouri Gov. Jay Nixon

Former Secretary of Agriculture and former Rep. Mike Espy (D-MS)

Sen. Dick Durbin (D-IL)

Chicago Mayor Lori Lightfoot

Former Sen. Joe Donnelly (D-IN)

Arizona House of Representatives Democratic Leader Charlene Fernandez

And it’s not just prominent politicians who have lined up behind Biden. Powerful unions in Michigan, Mississippi, and Missouri — all states that will be holding primaries on Tuesday — have also endorsed Biden.

Unite HERE! Local 24 and United Food and Commercial Workers (UFCW) Locals 876 and 951 in Michigan; UFCW Local 1529 in Mississippi; and UFCW Locals 655, 88, and 2 in Missouri have all thrown their support behind Biden in the past week, according to The Hill. They’re a formidable group of workers that come from a wide range of backgrounds — and their unions’ infrastructure could go a long way toward helping get out the vote for Biden. That will be key for him in Michigan, a state in which Sanders has powerful surrogates and that the senator is counting on in order to come back in the delegate count.

Sanders won a key endorsement himself this weekend: that of former presidential candidate and civil rights icon the Rev. Jesse Jackson.

“With the exception of Native Americans, African Americans are the people who are most behind socially and economically in the United States and our needs are not moderate,” Jackson said in a statement Sunday. “A people far behind cannot catch up choosing the most moderate path. The most progressive social and economic path gives us the best chance to catch up and Senator Bernie Sanders represents the most progressive path. That’s why I choose to endorse him today.”

Later Sunday morning, Sanders appeared on CNN’s State of the Union and said Jackson’s endorsement would be “a real boost” in his campaign.

“What Rev. Jackson understands is that we have to move aggressively to wipe out all forms of racism in this country and we need an economic agenda that speaks to the needs of working people, not just the billionaire class,” Sanders said.

Sanders already had the endorsements of many prominent progressive lawmakers, including Reps. Alexandria Ocasio-Cortez and Rashida Tlaib — with Jackson, he may have gained an endorsement meaningful to a constituency that has voted overwhelmingly for Biden in contests so far: older black voters.

Even with Jackson’s endorsement, however, the next few contests are likely to be an uphill battle for Sanders, particularly since party leaders appear to be increasingly coming out in support of Biden. And the Biden campaign has shown its ability both on Super Tuesday and in South Carolina to turn last-minute endorsements to its advantage.

In South Carolina, Democratic Rep. Jim Clyburn’s endorsement boosted Biden’s already strong hold in South Carolina, and he won nearly 50 percent of the vote there. His decisive victory in this Southern state helped fuel his wins on Super Tuesday — and the eleventh-hour endorsements of Buttigieg, Klobuchar, and O’Rourke likely also played a role, particularly in Klobuchar’s Minnesota and O’Rourke’s Texas, both states in which Biden did better than expected.

It is true that a Clyburn endorsement carries far more weight than the endorsement of just about any other political figure, but the volume and breadth of Biden’s endorsements in recent days can only help his chances of strong performances throughout March.