California Sen. Barbara BoxerBarbara Levy BoxerThe ‘bang for the buck’ theory fueling Trump’s infrastructure plan Kamala Harris endorses Gavin Newsom for California governor Dems face hard choice for State of the Union response 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 FeinsteinFormer US attorneys urge support for Trump nominee The Hill's Morning Report — Sponsored by PhRMA — Republicans see some daylight in midterm polling Senate panel clears bill to bolster probes of foreign investment deals 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.