Abstract: In Printz v. United States, a narrow majority of the Supreme Court continued the revival of constitutional federalism as a constraint on national power begun in New York v. United States. Professor Jackson concludes that Printz's categorical rule prohibiting federal directives to state employees is not well supported by historical or functional considerations but argues that courts should enforce milder federalism-based limits on national legislation. Judicial enforcement serves rule of law purposes, insisting that Congress recognize that it is constrained by law, and reinforces the constitutional role of the states. Although values such as liberty, participation, competition, and choice can be promoted at different times by different levels of government, securing the constitutional position of states helps preserve their governments as alternative locations of power and politics in which members of different groups can participate, crossing over otherwise important cleavages. The Article argues that judicial enforcement of two kinds of requirements is appropriate: first, with respect to federal regulation of private activity as in Lopez v. United States, that there be a considered connection, consistent with the Necessary and Proper Clause, between the legislation and an enumerated power; second, that the federal government not interfere with the states' constitutionally required legislative, executive, and judicial functions, an understanding that supports a strong presumption against legislative commandeering, and calls for a more nuanced approach to executive commandeering than in Printz. Finally, the Article argues that stability in sustaining a sufficiently principled law of federalism-based limits on national power can be better achieved with more flexible (rather than categorical) standards, given the dynamic and pragmatic character of successful federalism.