On preemption grounds, reports the Washington Post:
The Justice Department filed suit Tuesday against Arizona, charging that the state's new immigration law is unconstitutional and requesting a preliminary injunction to stop the legislation from taking effect.
The lawsuit says the law illegally intrudes on federal prerogatives, invoking as its main argument the legal doctrine of "preemption," which is based on the Constitution's supremacy clause and says that federal law trumps statestatutes. The Justice Department argues that enforcing immigration laws is a federal responsibility.
But the filing also asserts that the Arizona law would harm people's civil rights, leading to police harassment of U.S. citizens and foreigners. President Obama has warned that the law could violate citizens' civil rights, and Attorney General Eric H. Holder Jr. has expressed concern that it could drive a wedge between police and immigrant communities.