In this paper, we discuss the relation between social movements, public opinion, and political alliances with respect to the impact of movements on public policy. We first discuss the existing literature and sketch three broad models of the role of public opinion and political alliances (or the absence of such role) in facilitating the task of social movements in producing policy change: the direct-effect model, the mediated-effect model, and the joint-effect model. We test empirically each of this three explanations by means of time-series analyses of the mobilization of ecology, antinuclear, and peace movements in the United States between 1975 and 1995. The results show, first, that the three movements did not have a substantial impact on public policy, confirming that the direct-effect model has little explanatory power. Second, the mediated-effect model, too, is not supported by the empirical evidence, both in its public opinion and political alliances variants. Third, the joint-effect model is that which fits our data the best.