Abstract: The question of legal legitimacy, liberally understood, is that of whether anyone has a morally justified complaint about impressment into compliance with a particular law or set of laws. A good constitution may possibly contribute towards a liberally legitimate practice of national politics. This essay is concerned with one way in which a constitution, no matter how excellent, cannot thus contribute. We cannot use judgments regarding constitutionality to bridge the gap that must always exist between judgments regarding the validity of controversial laws and judgments regarding their legitimacy. We cannot hope to solve in that ’contractual’ or ’procedural’ way the riddle of government by consent in modern social conditions of deep ethical plurality and conflict, because we cannot successfully combine a proceduralist idea of constitutional legitimacy with a content-based conception of the binding virtue for constitutions. [O]ur exercise of political power is proper and hence justifiable only when it is exercised in accordance with a constitution the essentials of which all citizens may reasonably be expected to endorse in the light of principles and ideals acceptable to them as reasonable and rational. This is the liberal principle of legitimacy.