This article examines the formative influence of the organizational field of religion on emerging modern forms of popular political mobilization in Britain and the United States in the early nineteenth century when a transition towards enduring campaigns of extended geographical scale occurred. The temporal ordering of mobilization activities reveals the strong presence of religious constituencies and religious organizational models in the mobilizatory sequences that first instituted a mass-produced popular politics. Two related yet analytically distinct generative effects of the religious field can be discerned. First, in both cases the transition toward modern forms of popular mobilization was driven by the religious institutionalization of organizational forms of centralized voluntarism that facilitated extensive collective action. Second, the adoption of different varieties of the same organizational forms led to important divergences. The spread in the United States of societies for moral reformation—in contrast to their non-survival in Britain—steered popular politics there towards a more moralistic framing of public issues. These findings indicate the importance of the organizational field of religion for the configuration of modern forms of popular collective action and confirm the analytical importance of religion’s organizational aspects for the study of collective action.