This research aims to interpret the legal-political work of Jürgen Habermas in light of the problems and challenges indicated by Carl Schmitt. Initial hypothesis, confirmed later, affirms that Schmitt s Weimar work has a negative model function in the Habermasian theoretical perspective. The present work places itself in the interdisciplinary field of philosophy of law and political theory. The thought of the political and the normative conditions of democracy interweave into a certain concept of philosophy of law - as a thought that does not allow itself to forget the issue of legal legitimacy. It works yet in the contemporary register. Requirements, problems and crises of the current world are those which guide the readings of Schmitt, the comprehension of the Weimar experience, and of the fundamental elements of the work of Habermas in the 90 s.