Open primary

An open primary is a party nomination election where any registered voter may participate, regardless of party registration. Voters generally choose one party's ballot in the privacy of the voting booth and cannot vote in both parties' races in the same primary.

States differ in the details. In some, the voter simply asks for a party's ballot; in others, the voter's choice may be recorded publicly or the state may require a party declaration at the polls. The system contrasts with a closed primary, which only allows voters registered with the party, and with semi-open or semi-closed systems that allow unaffiliated voters to choose. It is also separate from a top-two primary, where all candidates appear on one ballot.

Supporters argue that open primaries boost participation by independents and may nominate more moderate candidates. Opponents, including party organizations, say they allow "crossover" voting, in which supporters of one party try to influence or sabotage the other party's nomination, and that parties, as private associations, should choose their own nominees. The Supreme Court held in California Democratic Party v. Jones (2000) that states cannot force parties to accept a blanket primary. Studies show limited evidence of strategic crossover.