This article examines the dimensional structure of roll‐call voting in the Senate. Using an unfolding analysis based on interest group ratings, a single ideological dimension is found. The robustness of this dimension is explored by examining how well it explains a series of roll‐call votes on foreign policy issues. As anticipated, the ideological dimension predicts these votes better than do other baseline models. This provides strong support for unidimensional theories of congressional voting behavior. Additionally, it suggests that while foreign policy may be substantively different from domestic policy, it is evaluated on the basis of a common ideological dimens