One-Party States and Legislator Extremism in the US House, 1876-2012

Do party strongholds exacerbate partisan extremism? Using over 140 years of election data and roll call votes I find that, on average, members of Congress elected in states where their party overwhelmingly dominates tend to moderate from their national party's ideology when compared to members elected in more competitive settings. I argue that heavily one-party states have often been engendered by widespread agreement over locally salient issues, group affect, or imbalanced party organization, not because the electorate consistently agrees with one national party. Consequently, representatives have incentive to run under the dominant party label but must respond to diverse electoral pressures that align to both national parties. While scholars prominently observe one-partyism in the Democratic South, I observe that the plains and rural west, particularly between 1896 and 1932, can be characterized as one-party Republican, too. Perhaps surprisingly, state party strength since the 1990s, in a historical perspective, is quite balanced.