Much of the literature on social movements centers on cyclical theories of political opportunity. While such work lays an important foundation for understanding contentious politics, it fails to fully integrate movements as actors in the American political system and public policy process. As such, the ways movements exercise power in the American political system, and the ways that power is constrained, are often not clearly conceptualized. This dissertation argues that movements exercise political power in the US in three distinct but overlapping ways; pluralist interest group power, plebiscitary opinion power, and disruptive contentious power. Through public law and empirical analyses, it shows that opportunities to exercise these types of power are limited by three patterns of American Political Development; insiders building structural constraints such as tax and campaign finance laws, political inflation caused by the expansion of political resources such as campaign spending, and institutional thickening that commits government resources to existing issues and limits slack resources for new issues. Case law analysis shows that the Supreme Court’s First Amendment doctrines on tax law, campaign finance law, and time, place and manner restrictions disadvantage movements. Empirical analysis of nonprofit tax filings shows that movements have increasingly relied on apolitical organizational forms such as charities. Analysis of protest news reports shows that policing policies have reduced confrontations between police and protesters in ways that lower this visibility of movements. Analysis of congressional hearings and public laws shows that an increasing share of government activity is devoted to administering existing policy commitments. The dissertation concludes that emerging constraints increasingly limit movement power in the future of American politics. As such, this project suggests that declines in social movement influence since the 1960s may not be a cyclical phenomenon, and that political outsiders must learn to adapt to a closed political system. Movement cases considered include LGBTQ Rights, Animal Rights, Disability Rights, and Antiabortion.