Although assertions that 'domestic politics matters' are common, it is not always recognized that theorists can assign a range of different roles to domestic factors. This article seeks to demonstrate that much research on domestic variables overlooks a potential role of social protest. Focusing on studies of cooperation and arms control, this article shows that they frequently either treat domestic politics only as a source of constraints that explain failures of cooperation, or view the public as purely reactive to efforts by state leaders to use international cooperation to blunt the impact of domestic problems. An analysis of US willingness to enter strategic arms talks during the Cold War suggests that this view of domestic politics is too narrow. A multivariate statistical analysis, supplemented by case-study evidence, shows that protest against nuclear weapons was a significant source of US decisions to seek strategic arms control. This suggests that theories that incorporate domestic factors need to allow for the possibility that public activism can contribute to changes in state preferences in the direction sought by activists, including a preference for security cooperation.