The conventional wisdom tends to treat constitutional arrangements, such as the allocation of foreign affairs powers, as efficiency enhancing constraints that yield benefits for all societal actors. This Article argues, on the contrary, that partisan actors can often manipulate the scope of the foreign affairs powers to achieve narrow ideological or electoral objectives, often at the expense of the political opposition. The issue of interpretative choice in foreign affairs powers usually involves the outcome of the struggle between right- and left-leaning groups in which each side attempts to increase constitutional constraints on issues that favor the opposition, and relax constitutional constraints on issues that benefit their favored constituencies. Using this framework, this Article analyzes how postwar partisan conflict between Republican and Democratic leaning constituencies on issues like human right treaties and war powers has both spawned and restricted the scope of foreign affairs authority in the United States. In the early post-World War II era, when the expansion of the national security and the welfare states was viewed as complementary, progressives favored greater presidential flexibility in both war-powers and human rights treaty ratification. But after the Vietnam War, when national security and social welfare became increasingly viewed as substitutes, progressives favored less presidential flexibility in war powers but not in human rights treaty ratification, whereas conservatives had an opposite set of institutional preferences. This postwar trajectory has continued through the Obama administration. Finally, this Article concludes by critically examining the normative implications of using increased judicial oversight to counteract the effects of foreign affairs partisanship.