The Geographic Dimensions of Electoral Polarization in the 2004 U.S. Presidential Vote

The 2004 U.S. presidential election was one of the most divisive in recent history (Pew Research Center 2004). The divisions in the electorate are popularly seen as the culmination of a process of political polarization underway since the 1970s (e.g., Frank 2004), and are epitomized by the now-ubiquitous map of the United States which shows swaths of red (i.e., majority Republican) states in the center of the country surrounded by blue (i.e., majority Democratic) states on the east and west coasts and in the north central region. In this chapter we investigate the geographic dimensions of political polarization in the United States through the lens of the 2004 election. We elucidate the principal contours of the divisions in the electorate, and characterize the manner in which the effects of the correlates of voting behavior cluster regionally. We take an ecological approach, using spatial econometrics to estimate the interregional divergence in the influences of the characteristics of populations and places on the odds of the Republican vote. To this end we employ aggregated data on 3,106 counties in the lower 48 states, which is the finest spatial scale at which both electoral returns and a variety of demographic and contextual variables are readily available. Our goal is to push the limits of ecological analysis in electoral geography. We first develop a theoretical framework in which geography plays a central role in electoral polarization. Our central hypothesis, which draws on themes in the political science literature (Johnston et al. 2004; Cho and Rudolph 2008), is that a number of social processes that operate at fine spatial scales tend to push individuals voters’ views into closer alignment with the ideological preferences of their geographically proximate majority – a phenomenon we call “localized entrenchment.” Drawing on the sociological literature on polarization (DiMaggio et al. 1996; Evans 2003), we circumvent the well-documented handicap of weak correlation between demographic attributes and ideology by employing a richer array of explanatory variables than prior spatial statistical analyses (e.g., O’Loughlin et al. 1994). We then apply spatial statistical techniques that exploit the spatial interrelationships among the electoral returns and our set of covariates, and find strong indications of entrenchment. Finally, we employ advanced methods to characterize the spatial heterogeneity in our estimated relationships – rather than re-estimate our aggregate statistical model on different regional sub-samples, we use geographically weighted regression (GWR). This technique enables us to exploit the spatial interdependencies among the entire universe of counties to estimate the fine-scale geographic variation in our covariates’ influences on the 2004 presidential vote, while simultaneously controlling for the underlying spatial distributions of the characteristics of people and places. The patterns of agglomeration in the resulting influences on voting behavior are consistent with our explanation of how local entrenchment might induce polarization of the electorate.