Income Inequality, Party Polarization, and Roll-Call Voting in the U.S. Senate

McCarty, Poole, and Rosenthal (2006) demonstrate that political parties—both in the electorate and in government—become more ideologically polarized during periods of high income inequality, while differences between the parties wane during periods of relative equality of incomes. I suggest that the processes described by McCarty, Poole, and Rosenthal at the national level are applicable to the American state electorates and their elected representatives. Using data on individual attitudes, state-level income inequality, state mass polarization, and U.S. senators’ roll-call behavior, I consider the possible effects of state-level income inequality on mass attitudes and the roll-call behavior of U.S. senators since the early 1960s. I hypothesize that (1) Democratic and Republican identifiers in the mass public should be more polarized in states with high income inequality, and (2) state mass polarization and state income inequality should be translated into polarized behavior by U.S. senators representing different political parties. My findings are generally consistent with these polarization hypotheses. Specifically, Democratic and Republican identifiers stake out divergent ideological positions as a function of state income inequality, and U.S. senators from states with high levels of income inequality are more polarized than other senators, primarily in response to state income inequality and greater constituency polarization that results from high income inequality.