California Sen. Barbara BoxerBarbara Levy BoxerHispanic civil rights icon endorses Harris for president California AG Becerra included in Bloomberg 50 list Climate debate comes full circle MORE (D) has backed Rep. Howard Berman (D-Calif.) in his primary battle with Rep. Brad Sherman (D-Calif.), citing an attack from Sherman's campaign that she said was outrageous.

Boxer had planned to steer clear of the primary between Berman and Sherman, two colleagues from Los Angeles who were pitted against each other by redistricting. But in a letter to Berman obtained by The Hill, Boxer announced she was endorsing Berman.

"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 said.

At issue is a direct-mail piece Sherman's campaign sent to voters featuring a photo of firefighters extinguishing a raging blaze. The mailer is an attempt to tie Berman to a utility company that has contributed to a super-PAC supporting his campaign.

"In 2010, PG&E's failure to maintain their gas line resulted in a massive explosion and eight deaths in San Bruno, Calif.," says the front of the mailer. "In 2012, PG&E is spending thousands to elect Congressman Howard Berman."

Boxer joins Sen. Dianne FeinsteinDianne Emiel FeinsteinFeinstein says she thinks Biden will run after meeting with him Trump judicial nominee Neomi Rao seeks to clarify past remarks on date rape Bottom Line MORE (D-Calif.), Gov. Jerry Brown (D-Calif.) and most of the state's Democratic congressional delegation in choosing Berman over Sherman in the primary.