You can change your party registration in most states. The states' laws are all different. Usually there is a cutoff - say 90 days before an election. After that, you cannot change it. This prevents people from "Pied Piper" voting... meaning, if I am a Democrat and I like candidate A, and I don't like Republican candidate B, I change my registration just before the primary to Republican and vote for candidate C. If enough people did this, they could get a different candidate on the Republican ballot for the general election in November. (Presumably candidate C is weaker than my preferred candidate A.)  Then, the idea is that you change back to your original party affiliation before the general election and vote for candidate A. He wins because you put a weaker candidate up against him.  If enough people did this, you could swing elections. My own state has something similar. You declare your party. They send you a primary ballot for that party only. You can only vote for a candidate in your party, not the opposite party. Usually there is a list of 2-3 guys in the same party vying for the same position. One of them is the clear leader and usually wins. The winner appears on the general election ballot in November, against the winner of the opposite party ballot. In the general election, you can vote for anybody, even people on the opposite party. Or you can write in your own choice. Not all states do this.