A lot of States have what's called "Closed Primaries" where they only let people that have registered with their party vote for who will be on the ballot. Ostensibly they do this so say Republicans can't mass join a Democrat primary and skew results by voting for a candidate with no motion that the Republican will easily beat in the general and vice versa.

In reality it's mostly a tool for party outreach and fundraising. If the Democrats know that 100 Democrats live in this neighborhood and they're like medium propensity voters they might send some volunteers to knock doors in that neighborhood, for example.

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