Abstract
The standard explanation for increasing party polarization in Congress is based on factors that would affect all policy issues simultaneously. We show that this has not happened. We examine the dimensionality of legislative choice in the House of Representatives, scaling eighteen issues in each Congress from 1965 through 2004. We detect considerable variability in issue dimensionality, an evolution in the structure of choice over time, and changes in the relationship between party unity and issue dimensionality. Our findings suggest that polarization has occurred on an issue-by-issue basis, reinvigorating the debate over the role of policy substance in shaping congressional politics.
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