BOTH sides in our nation’s ongoing immigration disputes are spinning the Arizona v. United States1 ruling as a victory—plus an occasion to appeal to supporters for more money to battle through the fall election cycle and the predictable next rounds of litigation and legislation. It’s the federal side, however, that has the better claim to success.

To be sure, the Court unanimously sustained—against a facial challenge—the “show your papers” provision that had drawn the most public attention. But the 5-3 majority (with Justice Kagan recused) did so with warnings about the provision’s implementation and the likelihood of future litigation.2 More importantly, the Court upheld a preliminary injunction against three of the four currently contested sections of Arizona’s restrictive immigration law, known as SB 1070.

In reaching that result, the majority warmly reaffirmed a constitutional doctrine, known as obstacle preemption, that will favor the federal government’s interests in a wide swath of future cases. It also strongly endorsed the primacy of the federal government in immigration control, in the face of a stunningly vitriolic dissent from Justice Scalia asserting the sovereign exclusion powers of the states. And it rejected a “mirror-image” theory propounded by SB 1070’s proponents that promised much future state legislative mischief.

The majority started with Section 3 of the Arizona law. That section imposed misdemeanor penalties, up to thirty days’ jail time, on persons who have not complied with federal alien registration provisions. A classic preemption case from 1940, Hines v. Davidowitz,3 had struck down a Pennsylvania state registration law, finding it in conflict with the then-recently-enacted federal scheme. Arizona argued that its law is different, because it simply mirrors the federal obligation, punishing only those who could be punished by federal authorities, instead of creating a separate set of substantive requirements. This intuitively appealing proposition masks serious complications, and the Court saw through it. The majority ruled that even the addition of minor state penalties can disrupt “the careful framework Congress [has] adopted.”4 Shaping a specific penalty structure and its enforcement machinery, while leaving its invocation in the hands of designated federal agencies, can be as significant a congressional policy decision as are substantive rules.

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