Community organizing is one of the most legitimate forms of collective action in the United States today, fostering the participation of urban working classes that are structurally excluded from the political field. And yet, the conditions of possibility of such socially unlikely participation have received little scholarly attention. Based on an ethnographic and sociohistorical inquiry conducted in Chicago, the historic birthplace of that repertoire of collective action, my goal in this dissertation is to address this gap by focusing on the group of professionals, called community organizers, who make popular and lay political participation and representation possible. The central paradox here is that, contrary to what the literature in political sociology usually argues, these professionals refuse to speak on behalf of the mobilized community, actively stepping back behind the spokespeople they select and train. What does the study of this role say about the links between processes of institutionalization, professionalization and politicization/depoliticization? In order to make sense of the original dissociation between the professional’s role and that of the spokesperson and understand how it affects the division of political work, the dissertation shows how the role of “leader-maker” has emerged, taken shape and been legitimized from the 1970s onwards, at the junction of, on the one hand, the reform-minded community organization tradition dating back to the early 20th century, and on the other, the legacy of the contentious politics of the 1960s and 1970s. I then shift the focus to what this hybrid role, where claims of professional expertise and mobilization and politicization cannot be disentangled, actually looks like in terms of daily practices developing popular representation. These practices occur within a space of political intermediation broadly shaped by networks of interdependencies with other competing sectors (the political and philanthropic fields, the “space of social movement”) which are beyond the lay spokespeople’s reach. By looking at organizers’ individual trajectories, however – from their social dispositions towards commitment to the actual incorporation of this pragmatic practical sense and the ways individuals can stay in the field or exit the role towards other career opportunities – the research shows that becoming an organizer can confirm or initiate dynamics of individual politicization.