Typically you have to be registered in either party in order to vote in their primary - which selects the candidate that runs in the November election. Cali has a weird thing - independents can vote Democrat, but not Republican . . . and Republican registrations got changed to Independent. This was actually done some time ago by the Republican party in Cali - because they were having independents vote for least qualified candidates in primaries, weakening their chances of winning in the November election.