Defenders of Arizona's harsh new anti-immigration law are going on the offensive: They are fanning out on TV, radio talk shows, and newspaper columns arguing that there is nothing nefarious about this law because all it does is help Uncle Sam enforce its existing laws. Even if this claim were true, it would be as constitutionally presumptuous as Arizona dispatching state troops to help the federal government fight the Iraq war.

Brit Hume, a Fox News commentator, who had previously called the law “somewhat draconian,” came out swinging in its favor—declaring that his initial characterization was completely wrong and the law is actually “totally sensible.” Meanwhile, the National Review crowd, which has yet to encounter an anti-immigration law that it doesn't like, put its full weight behind Arizona right from the get-go. In this it seems to be marching in lock step with the ultra-restrictionist Center for Immigration Studies (CIS) whose founder, Mark Krikorian, is a regular contributor to the magazine.

CIS advocates a moratorium on all immigration—illegal and legal, unskilled and skilled. And it pursues its agenda with as much finesse as Detective Clouseau displayed when pursuing criminals in Pink Panther movies. Two years ago, it released a study arguing against relaxing U.S. immigration restrictions on grounds that this would raise global greenhouse gas emissions. (Really! Check out the link.)

Even before the ink had dried on the Arizona law, CIS issued talking points insisting that it was nothing more than a mirror image of the federal law—a claim that Byron York, National Review's former White House correspondent, immediately repeated in his column. “Contrary to the talk [of the law's critics],” York declared, “it is a reasonable, limited, carefully crafted measure...that went to great lengths to make sure it is constitutional.” Likewise, National Review editor Rich Lowry maintained: “Arizona seeks only to enforce the nominal immigration policy of the United States.”

But such talk failed even to reassure Arizona's own legislators, who moved—less than two weeks after the law was passed—to amend the law's more draconian provisions after civil rights groups threatened to sue on constitutional grounds. However, even with these changes, the law raises equal protection and federalism issues large enough to drive a Mexican truck through.

One of the most controversial aspects of the amended law is that it makes it a state crime for immigrants—legal and illegal—to step out of their house without their papers. Defenders claim that there is nothing Gestapo-like about this provision because immigrants are already required by federal law to carry their papers. What's more, they note, this law won't mean that cops will simply be able to stop anyone on the street and demand proof of legality. Interestingly, they made the same claim about the original law even though it required police officers to make little more than eye contact before launching a full-blown inquiry into someone's immigration status.

The amended law limits such inquiries to instances when cops make a lawful stop, detention, or arrest in the course of enforcing some other law or local ordinance. But including local ordinances as grounds for an immigration inquiry opens all kinds of tantalizing harassment possibilities for officials like Joe Arpaio—the notorious but popular Arizona sheriff who has made it his personal mission to root out undocumented aliens from the state by launching crime sweeps in Latino communities on the flimsiest of pretexts.

Under the new law, Arpaio could troll Hispanic neighborhoods demanding the papers of anyone breaking, say, a local pooper-scooper law while walking their dogs. If they can't comply on the spot, he could haul them to a police station while their immigration status is checked. If it turns out that they are here illegally, they could be arrested, pending deportation. This means that the same pooper-scooper violation that would produce nothing more than a small fine for unaccented white folks—since they would not raise any “reasonable suspicion” that would justify an inquiry into their immigration status—could well result in a lengthy police encounter for Hispanic citizens or Hispanics with valid visas—and deportation for anyone who had overstayed their visa by even a day. “If this doesn't raise equal protection and Fourth Amendment concerns then what does?” asks Cecillia Wang, an attorney with the American Civil Liberties Union.

But the more serious constitutional problem with the Arizona law is that it illegitimately usurps a federal function given that, as with foreign policy matters, Uncle Sam has ultimate jurisdiction in setting national immigration policy. Professor Juliet Stumpf of Portland's Lewis & Clark Law School notes that courts have given states some leeway to set their own laws to deal with immigrant-related crime and other issues so long as their primary purpose is not to regulate immigration flows. But the Arizona law is not exactly subtle about what its true aim is. Its opening sentence reads: “The intent of this act is to make attrition [of the immigrant population] through enforcement the public policy of all state and local government agencies in Arizona.” In short, it is criminalizing immigration-related violations under state law not to deal with immigrant-related crimes—but to give state authorities expanded tools to drive out immigrants. Hence, even if the Arizona law is identical in content with federal law, notes Prof. Stumpf, it might still be overturned by courts on jurisdictional grounds.

But that's not the only way this law steps on federal toes. Congress' 1996 immigration reform act expressly forbade local participation in immigration enforcement without federal authorization and supervision. Except for Arizona, notes Muzaffar Chishti, Director of New York University's Migration Policy Center, he can't think of a single state or locality that has taken upon itself to enforce federal immigration law without first entering a memorandum of understanding with Uncle Sam. “Nothing in law is ever black and white,” comments Chishti, “but it is very unlikely that there is a judge out there who would uphold the Arizona law.”

The reason is that if states unilaterally start arresting undocumented aliens and dispatching them to the federal government for deportation, they will force the federal government to expend law enforcement resources on immigration when it might have other, more pressing, concerns such as, say, terrorism.

Ultra-restrictionists such as the Center for Immigration Studies who care about nothing else except hounding immigrants out of the country might think this is a good idea. But other cheerleaders of the Arizona law who have priorities other than an anti-immigrant jihad ought to rethink their support. They might well be setting Arizona up for an expensive fall in the courts without addressing the root cause of the illegal problem: The lack of avenues for unskilled foreign workers to legally work in this country. That's what they should be focusing on.

Shikha Dalmia is a senior analyst at Reason Foundation and a biweekly columnist at Forbes. A version of this column originally appeared at Forbes.