California Sen. Barbara BoxerBarbara BoxerTime is now to address infrastructure needs Tom Steyer testing waters for Calif. gubernatorial bid Another day, another dollar for retirement advice rip-offs 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.

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"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 FeinsteinThe case for protecting America's intelligence agency whistleblowers Senate confirms Trump's first lower-court nominee Feinstein: Comey memos 'going to be turned over' 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.