In the case of the new Arizona immigration law, the reductio ad Hitlerum occurred instantly.

Cardinal Roger Mahony wrote in a blog post, "I can't imagine Arizonans now reverting to German Nazi and Russian Communist techniques." The president of the Hispanic Federation said the law "reminded me of Nazi Germany." Cooler heads merely compared it to apartheid or 1960s-era civil-rights abuses.

And here I thought the tea partiers were befouling America's political discourse with their overheated words. They don't hold a candle to His Eminence or the assorted other hysterics decrying the rise of totalitarianism in the American Southwest.

Arizona's offense is to attempt to enforce the nation's immigration laws, in the absence of any serious commitment to do so on the part of the federal government or our political class.

The Arizona law makes it a state crime for aliens not to have immigration documents on their person. This sounds draconian, except it's been a federal crime for more than half a century - U.S.C. 1304(e). Has the open-borders crowd forgotten that it calls illegal aliens "undocumented" for a reason?

Police officers asking for papers may be redolent of old World War II movies. But consider the offending provision: "For any lawful contact made by a law enforcement official or agency of this state . . . where reasonable suspicion exists that the person is an alien who is unlawfully present in the United States, a reasonable attempt shall be made, when practicable, to determine the immigration status of the person."

Hitler would be crestfallen. This hardly reeks of extremism. It means the vast majority of requests for documentation will occur in the course of other police business, like traffic stops.

The police already have the power to stop illegal aliens, a power the Arizona courts have upheld; they already can ask about someone's legal status (the U.S. Supreme Court noted in 2005 that it has "held repeatedly that mere police questioning does not constitute a seizure" under the Fourth Amendment); and they already can detain illegal aliens. The Arizona law strengthens these existing authorities.

Will they be abused? Upon signing the law, Arizona governor Jan Brewer issued an executive order for a training program on how to implement it without racial profiling. No matter what her intentions, of course, it's unavoidable that Latino citizens will be questioned disproportionally under the law; nationwide, 80 percent of illegal aliens are Latino, and the proportion in Arizona must be higher.

Once millions of illegal aliens are in the country, there's no neat way to get them back out. It's much better to endeavor to stop them at the southern border, something Washington still refuses to do. During the last eruption of the national immigration debate, Congress passed a law mandating a fence along the border. The Bush administration bid it down to a high-tech "virtual fence." And the Obama administration has ceased constructing even that. If the federal government had been in charge of building the Great Wall, it wouldn't have been great or a wall.

It used to be that San Diego and El Paso accounted for most illegal entries. As the border became more secure at those points, Arizona became the hub. The state has an estimated 460,000 illegal aliens out of a population of 6.6 million. They impose countless millions of dollars in schooling, health-care, and incarceration costs, more than $1 billion a year in one estimate. Phoenix has become a kind of lawless Ellis Island, with smugglers holding migrants in "stash houses" there until they can be moved out into the rest of the country.

Arizonians needn't, and shouldn't, tolerate this. Critics accuse the state of unconstitutionally devising its own immigration policy. If it had unilaterally declared its border open to the poor, violence-plagued country to its south, this charge might have had force. Instead, Arizona seeks only to enforce the nominal immigration policy of the United States. Perhaps the federal government should try it sometime.