Critics have long attacked popular constitutionalists for offering few clues about how their theory might work in practice—-especially inside the courts. These critics are right. Popular constitutionalism—as a matter of both theory and practice—remains a work in progress. In this Article, I take up the challenge of developing an account of (what I call) popular constitutional argument. Briefly stated, popular constitutional argument is a form of argument that draws on the American people’s considered judgments as a source of constitutional authority—akin to traditional sources like text, history, structure, and doctrine. Turning to constitutional theory, I situate popular constitutional argument within contemporary debates over judicial restraint, living constitutionalism, popular sovereignty theory, and originalism. And turning to constitutional practice, I offer the interpreter a concrete framework for crafting popular constitutional arguments— cataloguing the various indicators of public opinion that have played a role in recent Supreme Court decisions. These indicators include measures associated with the president, Congress, state and local governments, the American people’s actions and traditions, and public opinion polls. Throughout, I use illustrative examples to show the various ways in which popular constitutional argument already operates at the Supreme Court—appealing to jurists from across the ideological spectrum. While this Article begins to explore how popular constitutionalism might operate inside the courts, much work remains