E.g. abortion rights, anti-LGBTQ, contempt for atheism, Christian nationalism, etc.
American here to confirm that @JoBo is basically right.
Some of the smaller parties have “Open Primaries” (which is to say that you can vote for who gets to represent the party in the real election, regardless of your party registration), but the big two (Democrats and Republicans) have “Closed Primaries” which means that if you want to vote for who will be the Dem/Rep candidate in the main election, you have to be a registered member of that party.
That system is like if I declared myself transwoman just to access the ladys room and see some boob and butt.
Boy, I sure do hate that analogy.
But it is the reason I’m registered with a major party vs the one I actually like; I can always have a voice in the one I like, and I want to be able to have my microscopic amount of influence on a larger party. It’s as much of a “have your cake and eat it too” in the less-than-optimal environment of American Politics as I can get.