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A longitudinal analysis of a 30-year period shows that the more heterogeneous a state's population, the more likely it is to have simultaneously a Democratic and a Republican senator. A path analysis linking constituency, competitiveness, party, and roll-call variables is tested on data from the 1970 census and the 93rd Congress. Geographic heterogeneity is related to diversity in the reelection constituencies of senators representing the same state. Reelection constituency is, in turn, related to party control of a state's Senate seats. While reelection constituency has a greater direct effect than geographic constituency on roll-call behavior, party has the largest direct effect. Constituency characteristics have a larger effect than party when direct and indirect effects are combined.
Bullock et al. (1983) studied this question.