It's Official! US vs. AZ!

The U.S. Justice Department on Tuesday filed a lawsuit challenging the constitutionality of Arizona's new law targeting illegal immigrants, setting the stage for a clash between the federal government and the state over the nation's toughest immigration crackdown.

The lawsuit filed in U.S. District Court in Phoenix argues that Arizona's law requiring state and local police to question and possibly arrest illegal immigrants during the enforcement of other laws such as traffic violations usurps federal authority.

"In our constitutional system, the federal government has pre-eminent authority to regulate immigration matters," the lawsuit says. "This authority derives from the United States Constitution and numerousacts of Congress. The nation's immigration laws reflect a careful and considered balance of national law enforcement, foreign relations, and humanitarian interests."

The government is seeking an injunction to delay the July 29 implementation of the law until the case is resolved. It ultimately wants the law declared invalid.