All active registered voters were mailed a ballot, which must be postmarked by Election Day and received by March 12. Early in-person voting was also available. The Associated Press expects the state to report additional votes each weekday as mail ballots continue to be counted. As of March 13, the A.P. estimates there were around one million ballots remaining to be counted.

California is one of five states on Tuesday holding primaries for the presidential race as well as its state primary. In state and congressional primaries, candidates from all parties are listed on one ballot, and the top two vote-getters advance to the general election. In the presidential primary, Republican voters were required to be registered with their party to vote for its nominee, but Democrats and unaffiliated voters could participate in the Democratic election.