In 2006, Congress passed the Adam Walsh Child Protection and Safety Act, one provision of which authorizes the federal government to civilly commit anyone in the custody of the Bureau of Prisons whom the Attorney General certifies to be “sexually dangerous.” The effect of such an action is to continue the certified person’s confinement after the expiration of his prison term, without proof of a new criminal violation. Six days before the scheduled release of Graydon Comstock—who had been sentenced to 37 months in jail for receiving child pornography—the Attorney General certified Comstock as sexually dangerous. Three years later, Comstock thus remains confined in a medium security prison, as do more than 60 other similarly situated men in the Eastern District of North Carolina alone. He and several others challenged their confinements as going beyond Congress’s constitutional authority and won in both the district and appellate courts. The United States successfully petitioned the Supreme Court to review the case. Cato, joined by Georgetown law professor (and Cato senior fellow) Randy Barnett, filed a brief opposing the government. We argue that the use of federal power here is unconstitutional because it is not tied to any of Congress’s limited and enumerated powers. The government’s reliance on the Necessary and Proper Clause of Article I, Section 8, is misplaced because that clause grants no independent power but merely “carries into execution” the powers enumerated elsewhere in that section. The commitment of prisoners after their terms simply is not one of the enumerated powers. While the government justifies its actions by invoking its implied power “to establish a federal penal system”—itself a necessary and proper auxiliary to certain enumerated powers—civil commitment is unrelated to creating or maintaining a penal system (let alone any enumerated power). Nor can the law at issue fall under the Commerce Clause, because civil commitment involves non-economic intrastate activity. As the Supreme Court recognized almost 150 years ago in Ex Parte Milligan, “[n]o graver question was ever considered by this court, nor one which more nearly concerns the rights of the whole,” than the government’s unconstitutional assertion of power against its own citizens. In this spirit, the Court should affirm the Fourth Circuit’s rejection of this blatant government overreach.