Closed primary

A closed primary is a nominating contest restricted to voters registered with the party holding it. A registered Republican votes in the Republican primary and a registered Democrat in the Democratic primary; voters with no party affiliation are excluded in the strictest version. Some states have "semi-closed" primaries that allow unaffiliated voters to choose a party's ballot.

Registration deadlines for party membership can come weeks before the primary, which means voters must decide early. Parties defend the system as a way to ensure that nominees are chosen by party members and to protect against strategic voting by outsiders, an idea supported by the Supreme Court's recognition of the parties' right of association, as in Tashjian v. Republican Party of Connecticut (1986), which let a party open its primary to independents.

Critics say closed primaries exclude a growing group of independent voters who still pay for elections, and may produce more partisan nominees. Supporters say that members of a party should decide its candidates. In many states, results can be significant because in areas dominated by one party the primary is the real contest. The system contrasts with an open primary and with a top-two primary.