This article deals with two attempts by French CGT union workers to organize casual workers in the car manufacturing industry and in a shopping mall. Efforts to organize among temporary workers involve an adaptation of demands and actions only partially coinciding with the traditional objectives of unionization. In order to reach temporary workers effectively and generate possible memberships, the union needs to create long-term links which leads to many tensions. Thus, the CGT teams which try and build temporary workers’ unions end up being poorly backed up by their own hierarchy and run out of steam quickly, either because the leaders are recruited at a higher level of the organization or because they are temporary workers themselves. The two case studies analyzed here give an insight into the way the CGT understands the widening scope of its activities, and how it implicitly orders its priorities.