California Sen. Barbara BoxerBarbara Levy BoxerKamala Harris on 2020 presidential bid: ‘I’m not ruling it out’ The ‘bang for the buck’ theory fueling Trump’s infrastructure plan Kamala Harris endorses Gavin Newsom for California governor 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 Emiel FeinsteinTrump: I hope voters pay attention to Dem tactics amid Kavanaugh fight Hillary Clinton: FBI investigation into Kavanaugh could be done quickly Graham calls handling of Kavanaugh allegations 'a drive-by shooting' 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.