Politics in the United States can now be characterized as an ideologically polarized twoparty system. The economy features increased income inequality. While the literature on comparative political economy has focused on the links between economic inequality and political conflict, the relationship between these trends in the United States remains essentially unexplored. Using National Election Study data from 1952 to 2000, we explore the relationship between income and voter partisan self- identification. We find that partisanship has become more stratified by income. We argue that this trend is largely the consequence of polarization of the parties on economic issues and the development of a two-party system in the South. The trend is much less a reflection of increased economic inequality. The partisanship results replicate for presidential vote choice. We also find that the two-party system has adjusted to remain competitive in spite of the large increases in real income in the last half of the twentieth century. If voters in 2000 were voting as if real average income were only that of 1960, partisanship would have swung strongly in the Republicans’ favor.