- Democratic congressional and state primaries: mixed
- Republican congressional and state primaries: mixed
- Democratic Presidential primary: mixed
- Republican Presidential primary: closed
- Voters may or may not choose to affiliate with a political party when they register to vote.
- 50% of registered voters in Idaho are unaffiliated.
Details of voting in congressional and state primary elections:
- The Democratic Party allows registered Democrats and unaffiliated voters to participate in their primary.
- The Republican Party only allows registered Republicans to participate in their primary.
- Idaho offers same-day voter registration, so an unaffiliated voter may affiliate with a party up to or on election day and vote on that party's ballot.
- Party affiliated voters seeking to change their affiliation must do so before the 10th Friday before an election.