Alaska - Open Primary: Parties select who may vote in their primaries. To vote in the GOP primary, a voter must be registered as a Republican 30 days before Election Day.
Georgia - Open: No party affiliation required at registration. However, on Election
Day, voters must declare an oath of intent to affiliate with the
particular party for whom they are voting on Election Day.
Idaho - Closed: Until 2011, all Idaho primaries were open. Independents intervened in a lawsuit brought by a faction of the Republican Party seeking to close their primaries. However, the GOP obtained a
declaratory judgment that mandating open primaries violated freedom of
association and was thus unconstitutional in Idaho Republican Party v. Ysura. Subsequently, the legislature passed a bill allowing parties to choose which type of
primary they use. Democrats have chosen a semi-closed primary;
unaffiliated voters may register a party at the polls on election day,
but they are bound to that party affiliation at the next election.
Massachusetts - Semi-Closed: Affiliated voters must vote in the primary of their party; however, unaffiliated voters may vote in either primary.
North Dakota - Closed: The only state without voter registration. To vote in the Republican
caucus you must have affiliated with the Republican Party in the last
general election or intend to do so in the next election.
Ohio - Closed: Voters' right to vote in the primary may be challenged on the basis
that they are not affiliated with the party for whom they are voting in
Oklahoma - Closed: Only voters affiliated with a particular party may vote in its primary.
Tennessee - Open: No party affiliation required at registration.
Vermont - Open: No registration by party. For presidential primary, voters must declare which ballots they want.
Virginia - Open: No party affiliation required at registration.