"Antitrust federalism, " or the rule that state regulation is not subject to federal antitrust law, does as much as-and perhaps more than-its constitutional cousin to insulate state regulation from wholesale invalidation by the federal government. For most of the last century, the Court quietly tinkered away with the contours of this federalism, struggling to draw a formal boundary between state action (immune from antitrust suits) and private cartels (not). But with the Court's last three antitrust cases, the tinkering has given way to reformation. What used to be a doctrine with deep roots in constitutional federalism is now a doctrine with close ties to the federal administrative state where courts sit in judgment of an agency's decision-making procedure.The new antitrust federalism conditions antitrust immunity not on the fact of state regulation but on the process of that regulation. Now, only regulation created by a politically accountable process is beyond the reach of federal antitrust suits, exposing vast areas of state regulation to new antitrust scrutiny. This Article argues that the new antitrust federalism is an improvement on the old, both because the old boundary model was unworkable and because the new regime addresses the "inherent capture " problems at the heart of modern state regulation. But this Article also warns that if the Court does not give accountability review real bite, it may have to abandon the new antitrust federalism and opt for a nuclear option that could portend the end of antitrust federalism altogether.