To revitalize union movements globally, labour scholars frequently prescribe social movement unionism. This union strategy adopts social change goals beyond member representation and contract negotiations and often requires allying with community organizations in pursuit of these goals. As a term, however, social movement unionism is often described in opposition to union organizational functions, such as member representation. This article challenges this organization-movement dichotomy by demonstrating the important influence of union organizational dimensions on the dynamics of social movement unionism. Analysis is based on case study research of labour union involvement in the 2006 immigrant rights marches in Los Angeles. Unions that participated in organizing these marches – thus, practicing social movement unionism – allied with large community organizations, preferred reform goals and advocated tactics perceived as effective. Such strategic decisions were informed by organizational considerations regarding members’ interests and unions’ long-term capacity for mobilization.