Indiana is a “modified open” primary state, meaning that voters do not officially declare a party affiliation on their voter registration forms and formal party affiliation is not required to vote in the primaries. Indiana election law stipulates that in order to participate in a party’s primary, a voter must have either voted for a majority of that party’s nominees in the last general election or must intend to vote for a majority of the party’s nominees in the upcoming general election.
In practice, however, secret balloting makes this provision of the law unenforceable. This creates an open primary system in which voters can participate in the primary election of their choice.