Congressional roll call voting has been highly structured for most of U.S. history. The structure is revealed by a dynamic, spatial analysis of the entire roll call voting record from 1789 to 1985. The space is characterized by a predominant major dimension with, at times, a significant, but less important second dimension. In the modern era, spatial positions are very stable. This stability is such that, under certain conditions, short run forecasting of roll call votes is possible. Since the end of World War II, changes in congressional voting patterns have occurred almost entirely through the process of replacement of retiring or defeated legislators with new members. Politically, selection is far more important than adaptation.
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