Using both a new data set of labor union appearances in congressional hearings and archival data on union organizational resources, this article analyzes factors that determine whether a labor union will be represented in congressional hearing testimony in a given year. Consistent with the expectations of resource mobilization theory, organizational resources are important predictors of participation in congressional hearings. For example, membership is an important predictor of testimony in hearings, as is the number of lobbyists on staff and the character of a union's primary industry. However, membership in the AFL‐CIO federation is negatively related to hearing participation, and some of the benefits of having a large membership base may be diminishing over time. Implications for the study of interest group politics and organizational political strategies are discussed.