The aim of this article is to understand the evolution of relations between the unions' field of action and the political parties' one. This question can be approached by the study of multiple engagements of militants. But this practice is currently in decline, and has to be completed by the analysis of the articulations betweens unions and political parties, particularly in their mutual conception of their field of action. The two main unions in France, CGT and CFDT, redefined recently in their discourses the limits of their field of action: their legitimacy to act in industrial relations is articulated to a new vision of this space as an autonomous one, unlinked from politics. On the otherwise, the two main political parties in France, PS and UMP, intervene more and more in unions' debates, assimilate essential elements of their proposals, and act as if they detent the "good model" of unionism. Theses discourses from political parties deprive unions of the autonomy they pretend give to them.