This Article introduces the theory of post-supremacy federalism to describe a structural condition in which federal law remains formally binding, but enforcement has durably receded or collapsed. Unlike frameworks grounded in intergovernmental friction or negotiated cooperation, this account begins from federal retreat—the cumulative erosion of enforcement capacity across judicial, administrative, legislative, and executive institutions. Post-supremacy federalism arises when three conditions are met: (1) federal law remains intact; (2) enforcement retreats; and (3) no formal devolution occurs. In this vacuum, states are not merely filling gaps. They are governing by default—under laws that still bind but no longer structure governance in a coherent or consistent way.

This Article maps the institutional dynamics behind this transformation, including judicial realignment, administrative collapse, legislative paralysis, and the rise of executive unilateralism. It then traces how this retreat manifests in four domains: drug policy, public education, environmental protection, and reproductive healthcare. In each, federal authority persists in theory, but meaningful enforcement has eroded—leaving states to construct and implement their own legal regimes without delegation, partnership, or protection. These domains are not newly decentralized. They are abandoned. Statutes like the Controlled Substances Act, Title IX, the Clean Air Act, and EMTALA remain binding, yet no longer coordinate or constrain state action. In their place, states exercise contingent authority—governing under conditions of legal precarity and enforcement asymmetry, exposed to federal snapback and forced to navigate an unstable landscape.

This Article does not propose reform. It offers a diagnosis. Post-supremacy federalism names the legal condition that emerges when federal authority recedes but does not relinquish control. Naming that condition is the first step toward understanding how governance now unfolds—and what may come next.