The Reciprocal Trade Agreements Act (RTAA) of 1934 has long been heralded as a simple institutional reform with revolutionary consequences: namely, by changing the trade policymaking process in the United States, the RTAA is held responsible for the dramatic liberalization in U.S. policy beginning in the 1930s and 1940s. This article takes issue with this conventional wisdom. I argue that the standard accounts—which emphasize the importance of delegation for overcoming logrolling in Congress or for facilitating reciprocity in international trade negotiations—fail to provide an adequate explanation for just how the institutional innovation was achieved and sustained in the face of protectionist opposition. I suggest instead that trade liberalization was driven by exogenous changes in party constituencies and societal preferences that had crucial effects on congressional votes to extend the RTAA authority and liberalize trade after 1945. The preservation of the RTAA program was symptomatic rather than causal; as a consequence, it may well be abandoned in the future. The evolution of U.S. trade policy has been, and will continue to be, powerfully shaped by changes in the preferences of societal groups and in the positions taken by parties on the trade issue.