Confronted with membership losses and declining bargaining power, trade unions have engaged in both political and organizational responses. A frequent type of organizational response has involved the creation of conglomerate unions, which bring together workers from various sectors and occupations. Pointing out a number of parallels between organizational developments in trade unions and political parties, this article analyses the emergence of conglomerate unions as a cause and consequence of changing conceptions of union democracy. Drawing on two in‐depth case studies conducted in France and Germany, the article examines how trade unions perceive their situation and how they define a reform rationale based on increasing their organizational ‘efficiency’ and ‘effectiveness’. In accordance with this rationale, unions engage in mergers and create larger conglomerates, thereby centralizing decision‐making bodies and professionalizing their staff. The reform of trade unions’ internal organization, in turn, affects unions’ capacity for interest aggregation and representation.