Feds sue to block Arizona illegal immigrant law

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 numerous acts 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.

The government contends that the Arizona law violates the supremacy clause of the Constitution, a legal theory that says federal laws override state laws. It is already illegal under federal law to be in the country illegally, but Arizona is the first state to make it a state crime and add its own punishment and enforcement tactics.