Upon conceiving constitutionalism on the scale of the nation-state as transparent and unproblematic, one may think global constitutionalism to be a mere utopia. On closer analysis, however, legitimation of nation-state constitutionalism turns out to be much more complex and contested than initially apparent, as becomes evident based on the contrast between liberal and illiberal constitutionalism. Upon the realization that nation-state liberal constitutionalism can only be legitimated counterfactually, the social contract metaphor emerges as a privileged heuristic tool in the quest for a proper balance between identity and difference. Four different theories offer plausible social contract justifications of nation-state liberal constitutionalism: a deontological theory, such as those of Rawls and Habermas, which privileges identity above difference; a critical theory that leads to relativism; a thick national identity based one that makes legitimacy purely contingent; and a dialectical one that portrays the social contract as permanently in the making without any definitive resolution. Endorsing this last theory, I argue that differences between national and transnational constitutionalism are of degree rather than of kind. Accordingly, it may be best to cast certain transnational regimes as constitutional rather than as administrative or international ones.