The mirror-image theory of cooperative state enforcement of federal immigration law is a phenomenon—one of the most wildly successful legal ideas in decades. The mirror-image theory proposes that states can enact and enforce criminal immigration laws that are based on federal statutes. The theory that it is unobjectionable for a state to carry out federal policy is the basis of Arizona’s Support Our Law Enforcement and Safe Neighborhoods Act—better known as SB 1070—and similar laws enacted in Alabama, Georgia, Indiana, and Utah. The same theory has provoked the introduction of bills in numerous other states and earlier but more narrowly focused immigration laws already in force in seven states. The mirror-image theory has succeeded not only in legislatures but also in the larger political culture: it has been embraced by dozens of U.S. senators and representatives, by policy groups, by private citizens, and by commentators including George Will, Sarah Palin, and the editors of the New York Post and the Washington Times.

The mirror-image theory is indeed appealing. But it is also fundamentally flawed. This Article, the first to subject the mirror-image theory to sustained scholarly scrutiny, demonstrates that the mirror-image theory fails to identify a legitimate source of state authority to legislate on immigration matters.

No one denies that Congress and the federal executive have exclusive authority over the substance and procedure of the admission, exclusion, and removal of noncitizens, documented and undocumented. This proposition was firmly established by a pair of Supreme Court decisions from 1876. The mirror-image theory does not challenge this deep-rooted idea head-on, but instead proposes that state legislative authority over immigration flows from federal cases and the provisions of the Immigration and Nationality Act that authorize states to assist in the enforcement of federal immigration law. Those sources, however, contemplate state assistance with enforcement only through arrests, and arrest authority does not imply the power to legislate or to prosecute. To the contrary, other provisions of the INA clarify that federal agencies have the exclusive power to make prosecutorial and administrative decisions after an arrest, as well as to create supplementary regulations.

The mirror-image theory rests on the erroneous premise that Congress has implicitly authorized state enforcement of federal immigration law. This Article argues that state enforcement would be unconstitutional even if it were explicitly authorized by Congress. First, the federal immigration power is exclusive and nondelegable. Second, criminal prosecution and immigration enforcement are executive powers that Congress cannot remove from the president and share with non-executive-branch officials. Finally, the Supreme Court has held that states cannot prosecute crimes that affect only the sovereign interests of the United States. Accordingly, state immigration prosecutions are irremediably unconstitutional.

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