Congressional primaries
As of May 2020, the states listed below utilized an open primary process for congressional elections. Variations are noted in the text alongside each state name. Note that the type of primary utilized for congressional elections in a given state may differ from the type of primary conducted for presidential races. And while there are open primaries, a political party could change the rules quickly. Additionally, in order to be on a party’s regional committees, you need to be registered with that party to hold office. So register, don’t simply vote in the primaries.
§ Georgia: On Election Day, voters must declare an oath of intent to affiliate with the particular party they are voting for.[8]
§ Hawaii: Voters may not participate in more than one party primary.[9]
§ Idaho: Democrats only
§ Illinois: Voters declare their party affiliation to a judge at the polling place.[10]
§ Indiana: According to FairVote, "A voter must have voted in the last general election for a majority of the nominees of the party holding the primary, or if that voter did not vote in the last general election, that voter must vote for a majority of the nominees of that party who is holding the primary. However, there is really no way to enforce this, and cross-over occurs often."[11][12]
§ Michigan: Though each primary ballot contains sections for two parties, voters may only vote in one party's section.[13]
§ Montana[17]
§ Ohio: A voter is not required to register with a party before voting in that party's primary. Instead, a voter selects his or her preferred party primary ballot at the polling place on Election Day.[11][19]
§ South Carolina: Voters must take an oath affirming that they have not voted in another party's primary.[20]
§ Tennessee: Voters must affiliate with a party at the primary polling location or declare their allegiance to the party.[21]