Through the example of family farmers’ participation in the Regional Program for Sustainable Development of Rural Areas (PDSTR), this article defends a scientific posture: open up the analysis of the participatory arenas by reintroducing them to the power relationships in policy-making. This research covers three political sociologies: the structures of political opportunity, the advocacy trajectories, and the repertoires of collective action of the participants, and policy analysis. It analyzes the institutionalization of a “hard core” of participants, characterized by their institutional advocacy and their transactional leadership, who format the policy projects from a technical and financial point of view. It also shows the constitution of an “advocacy coalition” that, in the end, defines the policy making and the contents of the PDSTR.