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The primary goal of this Article is to demonstrate that the interest in national unity does important, independent work in the law of vertical federalism. We have long been accustomed to treating union as a constitutionally operative value in cases involving the duties states owe one another (i.e. horizontal federalism cases), but in cases involving the relationship between the federal government and the states, the interest in union is routinely ignored. This Article shows that, across a wide range of cases relating to the allocation of power between the federal government and the states, the states are constrained by a duty to acknowledge their status, and their citizens’ identities, as members of a political community that is national in scope. These decisions are conventionally defended (by both courts and commentators) in supremacy-based terms. But I will show that they are rooted, instead, in an ethic of union.