Just a point - there are states with open primaries. Sometimes only one party has an open primary - which I believe is the case here in California. In Illinois it was all open primary. That means I do not have to be a member of the party in order to vote in the primary - but of course you can only vote in one primary! I would vote in whichever primary interested me.

Funny story with that...years ago my sister, who lived in Newark New Jersey was interested in the Republican primary. She asked for a Republican ballot at her polling place. Having never been asked for one before...they had a hard time finding one!

Randi, declaring a party means to get to vote in that party's election to decide who will be their candidate. In the primary there may be say ...five people who want to be the Republican candidate and four people who want to be the Democratic candidate. Rather than have all nine run against each other - the party holds a "primary" election to determine who will represent the party - one from each party. So it is a two step process.