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registered Republican
Most states have a party affiliation system. When you register to vote you can choose a party to be affiliated with, or choose to be independent. If you're affiliated with a party you can have a say in its internal workings. https://en.wikipedia.org/wiki/Voter_registration_in_the_United_States#Party_affiliation
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Your political party affiliation is the party that you choose to
associate with. Depending on your state, you may be asked your party
affiliation when you register to vote. You can change your
registration through your state election office. Before you do, know:You do not have to join a political party or reveal your party
preference when you register to vote. Not every state accepts or lists
a party affiliation on a voter registration card. The party
affiliation on your voter registration does not limit you to voting
for just that party. You can always choose to vote for a candidate
from any party in a general election, like a presidential,
congressional, or mayoral election. Your party affiliation is usually
only important in primary elections. Many states have "closed"
primaries. This means that you can only vote for your party’s
candidates in its primary election. Learn about the different types of
primary elections.
Shooter apparently a registered Republican and member of his local gun club
https://www.bbc.co.uk/news/articles/c3gw58wv4e9o