This Article identifies how the current spate of state and local regulation is changing the way elected officials, scholars, courts, and the public think about the constitutional dimensions of immigration law and governmental responsibility for immigration enforcement. Reinvigorating the theoretical possibilities left open by the Supreme Court in its 1875 Chy Lung v. Freeman decision, state and local officials characterize their laws as unavoidable responses to the policy problems they face when they are squeezed between the challenges of unauthorized migration and the federal government’s failure to fix a broken system. In the October 2012 term, in Arizona v. United States, the Court addressed, but did not settle, the difficult empirical, theoretical, and constitutional questions necessitated by these enactments and their attendant justifications. Our empirical investigation, however, discovered that most state and local immigration laws are not organic policy responses to pressing demographic challenges. Instead, such laws are the product of a more nuanced and politicized process in which demographic concerns are neither necessary nor sufficient factors and in which federal inactivity and subfederal activity are related phenomena, fomented by the same actors. This Article focuses on the constitutional and theoretical implications of these processes: It presents an evidence-based theory of state and local policy proliferation; it cautions legal scholars to rethink functionalist accounts for the rise of such laws; and it advises courts to reassess their use of traditional federalism frameworks to evaluate these subfederal enactments.