Voters may or may not choose to affiliate with a political party.
30% of registered voters in California are unaffiliated or independent.
Details of voting in congressional and state primary elections:
California has enacted Top Two, nonpartisan primaries at the state and local level; all registered voters receive the same ballot regardless of party affiliation, and the top two vote getters move on to the general election.
Voter registration closes 15 days before the primary election.