Abstract This article argues that the opposition between political and legal constitutionalism can be traced to a cleavage in what philosophers have called the ‘social imaginary’: the shared understandings that underpin social life. Since social imaginary understandings are by their nature nebulous and ill-defined, political and legal constitutionalism should not be thought of as competing theories or heuristic models, but—more abstractly—contrasting ways of imagining the political world. Drawing on historical and contemporary examples, I argue that my claim is supported by the way in which legal constitutionalism embedded itself as the governing idea in the United States and in France, and also by the failure of the ‘new Commonwealth model of constitutionalism’ to yield a genuinely distinctive alternative to political and legal constitutionalism.