At the crossing of research on political recruitment and socialization, this article analyses the process of the incorporation of the skills and know-how required of political staff among the members of the Mouvement des jeunes socialistes and the Jeunes populaires (UMP). Fieldwork enables to apprehend both the objective and subjective dimensions of these careers, and to underline their progressiveness and reversibility, as well as their shaping by the incitation and selection of the oldest members. Analyzing socialization to a political profession while it occurs - in practice and tacitly - enables thus to enter the “black box” of professional political competence and of the “practical sense” specific to the political field.