By exploring how early political investments in favor of a European Constitution have been turned into a legal enterprise to constitutionalize the European treaties, this article analyzes the changing role of legal elites in the genesis of a European transnational order. At first, legal activities Of constitution-making were closely linked to military issues and political mobilizations; later, the legal work of constitutionalization took a different path as a result of the process of differentiation of the European field of power and of the internal and contradictory logics of a newly created legal institution, the European Court of Justice (ECJ). By reconstructing the constitutionalization process, this article highlights the various types of elites then competing for the early definition of a European transnational order and, in particular, the capitals and representations of legal agents in the making of a Constitution for Europe.