LOS ANGELES — Hillary Clinton and Bernie Sanders are locked in a tight race in California, the nation’s most populous state and one that until recently seemed strongly in Mrs. Clinton’s corner, a new statewide poll has found.

The poll, released Wednesday night by the Public Policy Institute of California, showed Mrs. Clinton leading Mr. Sanders among likely voters, 46 percent to 44 percent — within the margin of error. A survey by the organization in March found Mrs. Clinton with a lead of 48 percent to 41 percent over Mr. Sanders.

The survey came as both Mr. Sanders and Mrs. Clinton have stepped up their campaign appearances here in anticipation of the June 7 primary. Mr. Sanders, after initially saying he would not advertise on television here — California is one of the most expensive states for television advertising, given its size and the number of media markets — took to the airwaves this week.

And Mrs. Clinton has quickly responded, with ads set to begin running on Friday in the Los Angeles, Fresno and Sacramento markets: one narrated by the actor Morgan Freeman, another highlighting Mrs. Clinton’s endorsement by the civil rights activist Dolores Huerta. The Clinton campaign will also run Spanish-language ads aimed at Latino voters, and translated commercials in Cantonese, Mandarin, Vietnamese and Korean.