This paper critiques ‘constituent power theory’ (CPT), which holds that the people, the constituent power, are the sole agents capable of legitimately creating a constitution, and that they necessarily act outside of and in tension with the institutions of democratic governance. We argue that underneath CPT’s cheery credentials of popular sovereignty and participatory democracy lays a foundation that is anti-pluralistic and elite-centric, and therefore undemocratic and illiberal. We make this argument by showing affinities between CPT and populism. First, we set out populism’s core normative tenets. Second, we reconstruct CPT as a legitimating theory and show that it endorses the same tenets, and thus embraces a politics in which viewpoint pluralism is rejected, citizens are treated unequally, and elites are empowered against democratic procedures in ordinary times. Third, we sketch out implications of our argument on participatory constitution-making and constitutional entrenchment.