Recent debates about popular constitutionalism and judicial supremacy have focused on the question of who interprets the Constitution. This Article reframes the debate by asking what legal sources courts apply to protect individual rights from government infringement. Throughout the nineteenth century, federal courts applied a mix of international law, statutory, and common law to protect fundamental rights and restrain government action. This Article uncovers the forgotten history of nineteenth century public law litigation. Professors Post and Siegel have advocated “policentric constitutional interpretation,” wherein the Supreme Court shares authority for constitutional interpretation with other actors. By analogy, this Article introduces the concept of “polymorphous public law litigation.” Under the polymorphous model, instead of fixating on constitutional law as the dominant public law discourse, courts apply international law, statutes, and common law—and occasionally constitutional law—to decide public law controversies. The Article demonstrates that nineteenth century federal courts applied a polymorphous model of public law litigation. During the twentieth century, a constitutionalized model of public law litigation supplanted the polymorphous model. In the constitutionalized model, courts rely primarily on constitutional law to decide public law cases. The process of constitutionalization exacerbated the tension between judicial review and popular sovereignty. When the Supreme Court applies constitutional law to decide a case, the Court does not merely decide the case; it also creates or modifies a legal rule that is not subject to revision by legislative majorities. In contrast, when the Court applies other types of law, Congress or state legislatures retain the power to modify the controlling legal rule. Hence, revival of a polymorphous model would help mitigate the te nsion between judicial review and popular sovereignty.