The Service Employees International Union (SEIU), the nation’s second-largest union, endorsed State Senate President pro Tem Kevin de León (D-Los Angeles) for U.S. Senate over incumbent Sen. Dianne Feinstein (D-CA) on Tuesday.

An endorsement from the SEIU, which backed both Barack Obama and Hillary Clinton, could give De León his biggest boost yet in one of the nation’s most closely-watched races.

SEIU just endorsed Kevin de Leon for Senate over Dianne Feinstein, the biggest boost he has had in his insurgent bid to topple the fellow Democrat who has been in the Senate for a quarter century. — Seema (@LATSeema) February 14, 2018

“SEIU just endorsed Kevin de Leon for Senate over Dianne Feinstein, the biggest boost he has had in his insurgent bid to topple the fellow Democrat who has been in the Senate for a quarter century,” the Los Angeles Times tweeted.

The SEIU also endorsed Democratic frontrunner Lt. Gov. Gavin Newsom for governor on Tuesday.

“SEIU California members are ready to work to see Gavin Newsom sworn in as California’s next governor,” Roxanne Sanchez, president of SEIU California, said in a statement. “We believe that California can show the nation the way forward to a society that values every person and makes real progress toward economic and racial justice. Gavin Newsom will be both a visionary leader and, more important, a partner of working people in accomplishing these goals.”

The SEIU noted in its endorsement of De León that his appeal to them was largely emotional, and that it was due to his taking the “time to meet with SEIU members and spen[d] a day walking in our shoes.”

The National Nurses United and its state organization, the California Nurses Association, is also backing De León on Monday because of his support for single-payer healthcare.

On Monday, former Los Angeles mayor Antonio Villaraigosa was endorsed by the California Police Chiefs Association and the Peace Officers Research Association of California.

Villaraigosa has caught up significantly to Newsom in the gubernatorial race in recent weeks.

California’s election laws reportedly prohibit Villaraigosa from listing his previous title as mayor of Los Angeles on the ballot. Villaraigosa was mayor of L.A. from 2005-2013.