This article proposes that the internal political organization of an interest group can shape its policy agenda. In doing so, it recommends that public policy research draw on scholarship on comparative political institutions to identify and theorize how alternative organizational rules, structures, and mechanisms can shape preference formation and expression. For example, confederal interest groups can amplify minority voices, whereas majoritarian groups can silence them. Contrasting cases of physician advocacy in mental health policy illustrate how the confederal approach to medical organization in France expanded the influence of a small group of public sector psychiatrists; while the majoritarian, “winner‐take‐all” approach to medical organization in the United States sidelined their American counterparts in favor of the private sector majority. These findings suggest that the politics of interest‐aggregation merit further investigation.