The American religious landscape is transforming due to a sharp rise in the percentage of the population that is nonreligious. Political and demographic causes have been proffered but little attention has been paid to the current and potential political impact of these “nones,” especially given the established link between religion, participation, and party politics. I argue that the political impact of nonreligious Americans lies in an unexplored subset of the nonreligious population called committed seculars. Committed seculars de-identify with religion, they adopt secular beliefs, and join organizations structured on secular beliefs. Using a unique survey of a secular organization, the American Humanist Association, I demonstrate that committed seculars are extremely partisan and participatory, and are driven to participate by their ideological extremity in relation to the Democratic Party. These results point to a long-term mobilizing dimension for Democrats and indicate the potential polarizing influence of seculars in party politics.