What role do public law and liberal constitutionalism play in an era of political populism? This article approaches this question by exploring the concept of constituent power in the light of recent constitutional developments in countries with populist governments. It attempts to outline and contrast conceptions of constituent power as inherent in liberal constitutionalist and populist thinking, respectively. While constitutionalists draw heavily upon Kelsenian normativism in framing the way political power is generated, populists juxtapose this with a concept of constituent power that is inspired by Carl Schmitt’s ‘decisionist’ view. The complacency of legality inherent in liberal constitutionalist thinking is susceptible to a populist challenge that draws attention to the necessity for the social embeddedness of any legal order. Populism, it is argued, exposes a core tension inherent in constitutionalism: How do constitutionalists reconcile their democratic aspirations with the simultaneous preclusion of certain political choices from the democratic realm? Populists can attack constitutionalism also because of the deficient conception of constituent power that underlies the latter. The article concludes that, where challenged by populists, public law can at some point no longer rely on its own force to defend itself. Its authority needs to be re-established from an extra-legal, pre-positive perspective. In an era of political populism, constitutionalist public law becomes a discourse that can challenge populism by means of the powerful reasons that inhere in the former.