The phrase "popular constitutionalism" most commonly refers to the role of the public—or perhaps its elected representatives—in framing answers to particular substantive questions of constitutional interpretation. This essay explores a different aspect of the popular constitution of the United States, one that is indifferent to particular substantive questions but that forms the basic structure in which most lawmaking takes place. The United States is not merely a federal system but one with concurrent federalism, in which many issues are regulated by both state and federal governments. This norm of regulatory concurrency became entrenched in the twentieth century even as the scope and depth of positive regulation of many social and economic issues proliferated. I argue that this concurrency norm itself is an important part of the public's constitution, in that it permits the public multiple outlets for lawmaking, in which different coalitions and actors may prevail at least temporarily. The federal judiciary has played, and will play, a central role in constructing and preserving this concurrent structure, and with it the multiple spaces for public lawmaking that exist. Where Congress has regulated at least part of a regulatory field, courts are often faced with the question of how broadly to trump, or preempt, state regulation of the same topic. After much judicial disagreement in the first 150 years of the nation's existence, by the New Deal era federal courts had embraced the concurrent idea of regulation as opposed to a more parsimonious theory of exclusivity arising from incremental federal regulation. By embracing this concurrency idea the Supreme Court enabled the proliferation and coexistence of both federal and state positive law in the later twentieth century. In the past decade, however, the Supreme Court and other federal courts have displayed more ambivalence on this point and have been demonstrably more willing to infer federal exclusivity even absent a clear Congressional statement, a development with troubling consequences for popular lawmaking.

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