I consider the role of unions in fostering electoral participation in the U.S. Using aggregate and individual data, I argue that turnout varies directly with the organizational strength of unions, not only through their ability to mobilize their own members per se, but also, and more importantly, through their ability to affect the rate of participation of voters in general. My results imply that unionization helps explain much of the post-1960 decline in voter turnout. The magnitude of the relationship is demonstrated to rival that of more conventional determinants of voter turnout such as education.