Rep. Howard Berman has already won more than two-thirds of the California Congressional delegation’s endorsements and picked up another big one today: Sen. Barbara Boxer.

In a letter to Berman today, the California Democrat said she could no longer stay out of Berman’s high-profile race with fellow Democratic Rep. Brad Sherman for the redrawn 30th district in the San Fernando Valley. Her decision to finally enter the fray came in reaction to a recent Sherman direct-mail piece that attempted to use a contribution from the PG&E utility company to a Berman-supporting super PAC as a way to tie Berman to the 2010 San Bruno pipeline explosion.

“Because of Brad’s campaign mailing, which outrageously tries to connect you to the San Bruno tragedy, I will no longer stay neutral in this race,” Boxer wrote today in a letter to Berman. “In the weeks and months ahead, I truly hope this campaign will take on a more positive tone and I would be proud to help people learn more about your stellar record of accomplishment and leadership.”

Boxer joins Sen. Dianne Feinstein, Gov. Jerry Brown and 23 House Democrats from California backing Berman. Sherman boasts two endorsements from his fellow delegation members, Reps. Judy Chu and Grace Napolitano.

Given the state’s new jungle primary system, this redistricting-forced Sherman-Berman battle for a safe Democratic seat will likely last past the June primary and into November.