We propose a political reform theory, a political and historical institutionalist argument that holds that shifts in political structures, partisan regimes and policy greatly influence movements. We appraise this argument, along with resource mobilization, political opportunity and media alternatives, by analyzing 600,000 articles in the New York Times and Washington Post that mention national U.S. social movement organizations (SMOs) in the largest 34 SMO industries across the twentieth century. We provide multivariate analyses of industry-level article mentions of SMOs and detailed analyses of the historical trajectories of coverage across the century. Although we find some support for major theories of movements and media influences, the political reform theory is strongly supported and outperforms standard political opportunity models. We conclude with suggestions to synthesize theories and for research on movement and media outcomes.