The US Department of Justice filed suit against the state of Arizona in federal court Tuesday, challenging the state's tough new immigration law requiring police to ask for ID from anyone they suspect of being undocumented.

The federal government will argue that federal law trumps the state statute and that enforcing immigration law is a federal responsibility. The Department has requested a preliminary injunction to delay enactment of the law, slated for July 29, arguing that the law's operation will cause "irreparable harm."

The DOJ's lawsuit is based on the preemption doctrine adopted by the Supreme Court under the Constitution's supremacy clause, which states that certain matters are of such federal character, as opposed to local or state, that only the federal government can act on them.

As straightforward as that sounds, legal analysts are split over possible outcomes.

“I agree with the argument that the federal government’s authority under the preemption clause will prevail over the Arizona state law,” says Robert Pugsley, professor of law at Southwestern Law School. “Otherwise we could have 50 states writing immigration laws and it would result in the chaos that the preemption clause was specifically created to prevent.”

Likewise, Kevin Johnson, Dean of the School of Law at University of California, Davis, says, “[we] have a clear situation where the federal government is having its powers intruded upon by the state government.”

But that view does not necessarily mean the suit will succeed. “There is good basis to believe the DOJ lawsuit may fail," says Hector Chichoni, partner at the national law firm Epstein Becker Green. "The Arizona law does not necessarily preempt the Federal government, rather [it] sets the state and local government as enforcers of what is already available, and on certain cases, mandatory under immigration law.”

Certain parts of the case are not black and white, including whether state police have authority to enforce federal immigration law, says Muzzafar Chisti, director of the Migration Policy Institute’s office at New York University School of Law.

A few weeks after its passage, state legislators modified the language dictating when a policeman could ask for papers from a suspected illegal immigrant. They added the stipulation that police could only do so if the person had already been pulled over for another offense, such as speeding. That action may have strengthened Arizona's case, argues Mr. Chisti.

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