Southern realignment, party sorting, and the polarization of American primary electorates, 1958–2012

Many scholars have argued that primary elections are an important factor in the polarization of the American Congress. Yet little research measures change in the policy preferences of primary electorates to evaluate the connection directly. We create the first explicit measures of the preferences of primary voters over the last 60 years using a Bayesian item-response theory model. Although the overall distribution of population preferences has changed little, the preferences of primary voters are now much more related to the party of the primary that they attend. We show that liberals are much more likely to turn out in Democratic primaries and conservatives are much more likely to turn out in Republican primaries. We estimate that the divergence of primary from general electorates is six times larger in 2012 than in 1958 owing to this “primary sorting”. This trend began with the emergence of the Southern Republicans. As the Republican party became viable, conservative Southerners switched to Republican primaries leading to a leftward shift in Democratic primary electorates. Nationwide, primary sorting began sometime after it began in the South. We speculate that Southern realignment played a clarifying role that contributed to subsequent sorting of primary electorates nationwide.