Sen. Dianne Feinstein, D-Calif., will face a Democratic primary challenger when she seeks her fifth full term in the U.S. Senate.
Kevin de Leon, California's Democratic Senate leader, said he will run against Feinstein, 84, according to CNN.
De Leon has started calling local officials and labor leaders in the state as he gets ready to campaign against Feinstein, who announced her plans to run for re-election on Monday.
California progressives have questioned Feinstein in recent months, especially after her August comments where she claimed President Trump could be a "good president." Feinstein has drawn the ire of supporters of Sen. Bernie Sanders, I-Vt., after she said Sanders "isn't a Democrat."
Feinstein also said recently that the Deferred Action for Childhood Arrivals program, put in place by former President Barack Obama in 2012, is on shaky legal ground and that Congress should pass a law to continue the program. Although Trump and Democratic leadership in Congress struck what was thought to be a tentative deal to help the Dreamers, the details of any agreement have yet to be ironed out.
De Leon has been a vocal opponent of the president in the state Senate, which has helped make California a "sanctuary state." They also passed a resolution calling on Trump "to publicly apologize to all Americans for his racist and bigoted behavior."
If de Leon runs, the two will take part in the state's jungle primary next summer. The top two finishers of that contest will face off in the general election in November. Feinstein has run only once in the jungle primary system in 2012 as the new format was put into effect in 2010 thanks to a ballot measure.
Feinstein has served in the Senate since 1992 when she won a special election two years before winning her first full term in the Senate in 1994. At 84, she is the oldest member of the Senate.