Partisan Polarization in Congress and the Historic Shift in the Geographical Bases of U.S. Political Parties

From the 1960s into the 21st century the American party system underwent two major transformations that were tightly interlinked—the ideological polarization of the parties and the regional shift of the party system. This paper uses a macrohistorical approach supported by an innovative method of analyzing long-term Electoral College and congressional election data to empirically demonstrate the two-way causal relationship between the ideological polarization of the parties and the unusually slow but eventually complete regional realignment of the U.S. party system. The ideological polarization so prominent since the 1990s actually began in the 1960s, first with intensifying ideological conflict in presidential races, which then set off a shift in the regional bases of the political parties, and which eventually culminated with more cohesive party voting and fierce ideological battles in Congress.