Over the past decade, the Supreme Court has been unusually active in striking down acts of Congress in the name of federalism. The Court's activism in this area is particularly striking since the judicial enforcement of federalism had largely been abandoned since the New Deal. The article offers a political explanation for the Court's federalism offensive. The federalism offensive can best be understood as a product of the Court taking advantage of a relatively favorable political environment to advance a constitutional agenda of particular concern to some individuals within the Court's conservative majority. The article elaborates each of these elements that have been supportive of the Court's actions, indicating the value of the new institutionalism to explaining judicial behavior and explaining why the Court's recent actions are distinct from the Court's actions leading up to the Court-packing plan and the judicial retreat of 1937.