Former Vice President Joe Biden Joe BidenJoe Biden looks to expand election battleground into Trump country Trump puts Supreme Court fight at center of Ohio rally Special counsel investigating DeVos for potential Hatch Act violation: report MORE endorsed Sen. Dianne Feinstein Dianne Emiel FeinsteinBiden leads Trump by 12 points among Catholic voters: poll Names to watch as Trump picks Ginsburg replacement on Supreme Court McConnell says Trump nominee to replace Ginsburg will get Senate vote MORE’s (D-Calif.) reelection bid in the face of a challenge from the left, her campaign announced Thursday.

Biden noted that the two worked together in the Senate, and praised Feinstein for her advocacy for California and “progressive values we share.”

“She’s tenacious. She’s accomplished. And she’s exactly who we need in the Senate to stand up to this Administration and its Republican allies in Congress. Dianne is a dear friend, and I’m proud to endorse her re-election,” Biden said in a statement.

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“I’m so honored that Joe has endorsed my re-election — his support is a big deal,” Feinstein said in a statement.

Feinstein, 84, has held her Senate seat since 1992, but faces a challenge in a Democratic primary this year. She failed to clinch the California Democratic Party’s endorsement last month.

Feinstein earned 37 percent of the delegates’ votes at the state party’s annual convention, while her main rival, state Senate leader Kevin de León (D), received 54 percent of the vote. Candidates needed 60 percent to win the endorsement.

De León has rallied progressives who are frustrated with Feinstein and feel she hasn't held President Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE accountable.

Some liberal leaders, including Sen. Bernie Sanders Bernie SandersJacobin editor: Primarying Schumer would force him to fight Trump's SCOTUS nominee Trump campaign plays up Biden's skills ahead of Cleveland debate: 'He's actually quite good' Young voters backing Biden by 2:1 margin: poll MORE (I-Vt.) have thus far declined to weigh in on the race.

The Democratic primary will be held on June 5.

Biden, meanwhile, is at the center of speculation over whether he intends to run for the White House in 2020. He has said he is open to the idea, but has not made a final decision.

The former senator and vice president has taken an active role in the 2018 midterms, campaigning hard for Democrat Conor Lamb ahead of his special election victory earlier this month in Pennsylvania's 18th District.