The Clean Air Act, like many federal environmental statutes, relies upon the cooperation of state environmental agencies for its execution and enforcement. If states do not cooperate, the Clean Air Act obligates the federal government to regulate in their stead as well as to impose potentially draconian sanctions. Specifically, the Act calls for the revocation of federal highway funds in noncooperative states. There are reasons to suspect that the Clean Air Act’s sanction regime is unconstitutional, particularly in the wake of National Federation of Independent Business v. Sebelius, which enforced a limit on the federal government’s ability to induce states to cooperate with federal programs. Most significantly, NFIB held that conditioning receipt of federal Medicaid funds on a state’s willingness to participate in a dramatic expansion of the program was unconstitutionally coercive. Combined with the Court’s prior holding in South Dakota v. Dole, NFIB raises questions about the constitutionality of the Clean Air Act’s highway fund sanctions and may open the door to challenges to other portions of the Act as well.