This article analyses the strategy of major Spanish trade unions in the context of economic crisis through the theoretical framework of political exchange. It focuses on a frequently ignored dimension of political exchange, namely the relationship between union leaders and the rank-and-file, which, under certain circumstances, can lead to a crisis of representation. Based on document analysis and semi-structured interviews, this article explores the strategic behaviour of Spanish trade unions and their relationship with the so-called 15M movement. It concludes that the ambiguous strategy of the major unions can be explained, not only as a result of their effort to preserve their institutional power and their political influence in the public sphere, but also as a response to the emergence of the 15M movement, which has made these organizations confront the threat of a crisis of representation.