LOS ANGELES—Voting in the nation’s most populous and delegate-rich state has already started, even though California’s primary isn’t officially for a month, forcing Democratic presidential campaigns to turn attention toward it immediately after the Iowa caucus.

More than 15.7 million California voters, three-quarters of the state’s electorate, are receiving mail ballots that started going out Monday, according to research firm Political Data Inc. In-person voting will begin in thousands of locations in 15 counties on Feb. 22, including Los Angeles, the state’s most populous county.

Given the high level of interest in the Democratic primary in the left-leaning state, observers expect record participation.

Pete Buttigieg, the former mayor of South Bend, Ind., and Vermont Sen. Bernie Sanders led in partial results from the Iowa caucuses following reporting delays.

California is awarding 494 delegates of the 1,991 needed to secure the Democratic presidential nomination this year. By moving up the official date of its contest to March 3, from June previously, the state has drawn much more early spending and visits from candidates who also are busy competing in the four states with February contests.