We interpret three strands of social movement theory (social threat, opportunity structure, and resource mobilization) to understand religious congregations' political activity related to same-sex marriage. Using a unique, national data set from the 2008 presidential election, we show the importance of the anti-same-sex marriage discursive opportunity structure (DOS) for facilitating, and occasionally constraining, congregational political activity. Other theoretical factors influence congregations under limited, although important, conditions. Our research develops a nuanced understanding of congregations' roles in social movements that prioritize institutionalized political activity and expands DOS theory in relation to local religious organizations.