This article examines the effect of union membership on civic and political participation in the late 20th century in the United States. We discuss why and how unions seek to mobilize their members and where mobilization is channeled. We argue that union membership affects electoral and collective action outcomes and will be larger for low socioeconomic status individuals. Statistical analyses find that union membership is associated with many forms of political activity, including voting, protesting, association membership, and others. Union effects are larger for less educated individuals, a group that otherwise exhibits low levels of participation. Union membership is not associated with outcomes distant from union political agendas, such as general volunteering and charitable giving, suggesting that unions generate political capital rather than generalized social capital.