Thousands of studies have examined party competition in the American states, finding significant consequences for voter turnout, policy adoptions, and more. Long-term patterns of party control have received less attention. Here, we reexamine the operationalization of party competition. We then update Klarner’s state partisan balance data to include state house and senate composition and gubernatorial vote share since the 1930s, adding—in light of the nationalization of American politics—presidential vote share and the proportion of Democrats in each state’s congressional delegation. After establishing a threshold for one-party dominance, we examine the frequency and duration of subnational party monopolies, highlighting regional variations in the relationship between the state and national measures and applying the index to voter turnout. Our analysis reveals that extended periods of one-party dominance – currently on the uptick – are the rule, not the exception, in the American states and are a phenomenon ripe for further exploration.
