The power to make constitutions (the so-called constituent power) is predominantly understood today as a legally unlimited power belonging to the people. This understanding sits uncomfortably with constitutionalism: the idea that public powers are legally limited. Would such a power not leave an indelible blemish on constitutions that are otherwise committed to constitutionalism? This article shows that this problem, which I call the Stain of Constitutionalism, stems from a misapprehension of what constituent power was originally understood to be. Focusing closely on the writings of Emmanuel Joseph Sieyès, Thomas Paine, and the Marquis de Condorcet, I demonstrate that, far from adopting it, these founding fathers of constituent power theory rejected the notion of unlimited constituent power. Instead, they defended a natural rights approach according to which constituent power is legally limited by considerations such as freedom and equality.