In this study, we analyze the voting records of U.S. senators on three recent international trade bills. Our objective is to assess the importance of ideology and constituent interest as forces that influence protectionist vs. free trade voting behavior. The methodology uses discriminant analysis supplemented with expert judgement. The first conclusion is that ideology is the more important force in generic protectionism issues, and local constituent interests are more important for commodity-specific issues. Second, we find that protectionism is especially associated with liberal ideology, high import impact, and low export dependence. Although the discriminant analysis outperforms expert judgement in classifying protectionist vs. free trade votes in two of the three bills studied, the best explanation is obtained by using both methods together.