Despite their decline, unions, and especially public sector unions, remain important civic and economic associations. Yet, we lack an understanding of why public sector union members voluntarily support unions. We report on a field experiment conducted during a 2017 Iowa teachers union recertification election. We randomly assigned union members to receive emails describing union benefits and measured effects on turnout effort (N = 10,461). Members were more likely to try to vote when reminded of the unions’ professional benefits and community—but not legal protections or political representation. A follow-up survey identified the specific aspects of professional identity and benefits that members most valued and why. In a context where union membership and support is voluntary among professionalized workers, our findings emphasize the possibility of training for fostering shared identities and encouraging support for public sector unions. Our results have broader implications for understanding the public sector labor movement in a context of legal retrenchment.