A good thing the parties don't have registered party affiliation lists they can x-verify the information against then. Of registered voters in the US, 47% have declared a party affiliation.
Party affiliation isn't the same as voting, it's a weird thing where what would normally be a private members organization is nationalised somehow.
Is that one public? Like all these news articles that name which party someone is registered for would imply such.
Party affiliation is public information, at least in Virginia.
In most states it's public. The Republicans would also have access to their own proprietary voter lists or databases, as well as publicly available voter registration records.