Harris, the only major Democrat in the Senate race so far, announced the endorsement of Los Angeles County Dist. Atty. Jackie Lacey on Tuesday.
Lacey was the second prominent Los Angeles African American endorsement that Harris rolled out in as many days. On Monday, City Council President Herb J. Wesson Jr. announced his support for Harris.
Wesson, a longtime political ally of Villaraigosa's, said one of the reasons he backed Harris was that she would be California's first African American U.S. senator. Harris' father is Jamaican and her mother was Indian.
Villaraigosa, who would be the state's first Latino in the Senate, would need broad support in the Los Angeles area to counter Harris' strength in the Bay Area. Harris was San Francisco's district attorney for eight years.
In a written statement, Lacey said Harris would "ensure that our state's public safety leaders and all Californians have a voice in the nation's capital."
Harris "has been an effective and aggressive chief law enforcement officer for the state, and has been a true partner with local law enforcement," she said.