The new Arizona immigration law, which allows police to stop people they suspect of being illegal immigrants, might never go into effect because of the many serious legal challenges it faces. A Tucson police officer already has filed a suit against the law and the Justice Department is reportedly considering a lawsuit.

Federal immigration law may trump the Arizona law because the federal government has broad constitutional authority over immigration, legal experts say. In addition, the Arizona law may violate the 4th Amendment's search and seizure requirements by allowing police to stop a person if the officer has "reasonable suspicion" that the person is an illegal immigrant. The state says it won't use racial profiling in enforcing the law, but critics say there is no way to avoid it.

"I can't imagine how the police could have reasonable suspicion that someone is an illegal alien," says Richard B. Kuhns, a law professor at Washington University. Kuhns wrote in an email that police might have probable cause "if the police know or think they know the person's status from prior contacts, if the person is fleeing from the border, etc." But without that information, he added, "there simply aren't facts that would give a police officer (only) reasonable suspicion about a person's illegal status."

Roger Goldman, law professor at Saint Louis University, agreed that the Arizona law was questionable. Courts have generally permitted stops based on reasonable suspicion for administrative purposes, but not criminal ones. "This statute seems for purposes of criminal law enforcement, which could trigger the need for probable cause," he wrote in an email.

But the Missouri law professor who helped Arizona write the new law says it will be upheld.  Kris Kobach of the the University of Missouri-Kansas City says that states can enforce laws against the same behavior that federal law criminalizes.  Kobach was an assistant to former Attorney General John Ashcroft.

The Constitution gives Congress the power to "establish a uniform rule of naturalization . . . throughout the United States." This gives Congress so-called "plenary power" in immigration matters, meaning that it can exert almost unlimited authority and preempt state laws.

In a legal analysis of the Arizona law this week, the Constitutional Law Prof Blog concluded, "The new law is almost certainly invalid under the Supremacy Clause."

Federal immigration laws "reflects Congress' judgment to completely occupy the field of immigration and naturalization," with a few exceptions," states the blog's analysis.

One exception is that local police are authorized to arrest illegal immigrants who have previously been convicted of a felony and deported. But the new Arizona law gives local police much broader authority than Congress allows, the blog concludes.

Courts also have recognized that local authorities can enforce traditional licensing laws. A previous Arizona law making it a crime to employ illegal immigrants was upheld as was a similar law in Valley Park denying or suspending business permits to those who employed illegal immigrants. But that kind of licensing exception does not save the new Arizona law, experts say.

Kobach, who defended Valley Park, disagrees.  He says that concurrent enforcement of the same legal requirements that the federal government imposes does not violate preemption.

Under the 4th Amendment, police have been permitted for decades to stop a person based on "reasonable suspicion," which is less than probable cause. These so-called Terry stops were upheld by the Supreme Court in Terry vs. Ohio in 1968.

"Reasonable suspicion isn't a very specific standard," says Kuhns, "but the court has consistently said there have to be objective articulable facts that support the suspicion. So either (1) properly interpreted it adds nothing to existing police power because there is no such thing as mere reasonable suspicion that a person is an illegal alien, or (2) if it becomes a basis for legal stops, that will mean that courts have gutted the reasonable suspicion test of all content."

Goldman wrote that courts "have permitted 1) roving patrol stops where there is reasonable suspicion of illegal aliens, and 2) permanent checkpoints without need for any suspicion at all, but always in context of federal INS agents." The theory, says Goldman, is that auto checkpoints and roving patrol stops are to make sure a person has a license, or a safety inspection, all for administrative purposes. The Arizona law is for criminal purposes, which may require the higher standard of probable cause, Goldman wrote.

The lawsuit filed by Tucson police officer Martin Escobar asserts that nothing about an illegal immigrant -- not the car he drives, the clothes he wears or the living arrangements he inhabits -- gives a trained officer the information needed to form a reasonable suspicion that a person is an illegal immigrant.

In addition to arguing the Fourth Amendment amendment and preemption issues, the suit also claims that the law violates the 14th Amendment's promise of equal protection of the law.

The Arizona legislature amended the law late in the week to address some of the criticisms.  The amended law states that police should scrutinize only people they stop or arrest, not any person who asks a question.  The legislature also prohibited stops based solely on race.  Critics called the changes cosmetic, adding that the law still violates the Constitution.