The liberal international legal order faces a legitimacy crisis today that becomes visible with the recent anti-internationalist turn, the rise of populism and the recent Russian invasion of Ukraine. Either its authority or legitimacy has been tested many times over the last three decades. The article argues that this anti-internationalist trend may be read as a reaction against the neoliberal form taken by international law, not least over the last three decades. In uncovering the intricacies of international law’s legitimacy crisis, the article uncovers the paradox of global constitutionalism: that its need to adopt a sectoral form of integration may cause a legitimacy gap/deficit because international authorities, resting their legitimacy primarily on instrumental grounds, may face problems in compensating for the legitimacy deficit caused by the erosion of domestic sovereignty and extending their legitimacy to non-instrumental grounds. This paradox has one necessary structural and two contingent content-related implications for domestic democracies: (1) it necessarily narrows down the regulatory space of nation-states; and this may in turn (2) impair democratic stability and solidarity, and (3) provide a fertile ground for populism. Drawing on Raz’s service conception, the article focuses on the interaction between international and domestic authorities and highlights the problematic aspects of the neoliberal constitutionalization of international law.