This study examines mobilizing mechanisms using a British community organization and a British trade union as exemplars. Although there has been substantial work on union revitalization on the one hand, and the emergence of alternative, community organizations on the other, no study has compared the challenges these organizations face in encouraging member mobilization. The findings illustrate how the trade union engages in a service-driven culture, cultivating instrumental commitment between the members and the union. The community organization, in contrast, engages in a relational culture and exemplifies a form of social commitment between the members and the group. As a result, different types of commitment and organizational cultures help explain why sustained member mobilization within a trade union is harder to achieve than within a community organization.