The Dynamics of Hidden Partisanship and Crossover Voting in Semi-Closed Primaries

Among US states with party registration, many allow the unaffiliated to choose either the Democratic or Republican primary. States with these semi-closed rules thus provide an option to voters with greater choice than registering with a single political party. Using the synthetic control method, I find that the introduction of semi-closed primaries is associated with growth in unaffiliated registration. However, the likelihood of unaffiliated registration is not even across the electorate in semi-closed states. I show that it is most common where a voter's party is not competitive and the access unaffiliated registration provides to the strong party's primary is valuable. Consistent with this instrumental motive, unaffiliated voters in semi-closed states use their freedom of choice to vote in the primary of the stronger party in the electorate. This leads to significant crossover voting among unaffiliated voters who do not identify with that party such as Democrats in red states or Republicans in blue states. These findings show the unintended consequences of electoral institutions and find primary crossover voting is more common under some circumstances than others.