The public events for Mr. Buttigieg were designed to show him being introduced to Latino voters and listening to their concerns. In Walnut, east of Los Angeles, Mr. Buttigieg received a warm introduction from the city’s 26-year-old mayor, Andrew Rodriguez. But Mr. Rodriguez clarified afterward that he hasn’t endorsed Mr. Buttigieg — he’s still thinking about Mr. Biden, too.

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“I just relate to Pete because he’s a young mayor,” Mr. Rodriguez said.

Mr. Sanders, who polls show has a clear advantage with Latino voters in the state, stopped at the Mexican border on Friday night before holding a rally on Saturday with Representative Alexandria Ocasio-Cortez of New York that drew 14,000 people to Venice Beach.

“For most candidates, California is a piggy bank,” Mr. Weaver said as crews cleaned up after the rally. Mr. Sanders, he said, “doesn’t just come here and go to the wine cave — sorry, I had to — and then fly out to Iowa. He actually comes here and talks to voters.”

Mr. Sanders and his campaign are also doing the sort of spadework his 2016 campaign didn’t. He has opened offices in heavily Latino regions of the state and is doing the sort of outreach to local elected officials he resisted last time.

Nick Carter, the national political outreach director for the 2016 Sanders campaign, said it was “challenging” to get Mr. Sanders on board back then. “He was giving his time to voters and not just currying favor with politicians,” Mr. Carter said.

One of the officials Mr. Sanders courted this year was Reggie Jones-Sawyer, a state assemblyman from South Los Angeles. Mr. Jones-Sawyer had been an enthusiastic supporter of Mrs. Clinton in 2016 — a Little Rock native, he has deep roots with the Clintons — and had in May endorsed Senator Kamala Harris of California for the 2020 presidential nomination.

In November Mr. Jones-Sawyer took a meeting with Mr. Sanders on the sidelines of the California Democratic Party State Convention. Ms. Harris was still in the race, but struggling. Mr. Sanders made the pitch, and Mr. Jones-Sawyer told Mr. Sanders he was his second choice.