Let’s leave aside, for the moment, the question of whether Arizona’s new immigration law is, as President Obama asserts, “misguided.” This newspaper happens to agree with the president on the need for a more capacious immigration reform that includes a path to citizenship even for those who came here without proper papers. Mayor Bloomberg has been particularly eloquent on this head. But we also comprehend that passions are running high and that there are serious individuals on both sides of the matter. The more newsworthy, and even explosive, question is whether what Arizona is doing is constitutional. Are we are headed for yet another confrontation that will expose the hard surface of our national bedrock?

We certainly hope so. Few powers are more clearly enumerated in the Constitution as belonging to the federal government than that which, in Article 1, Section 8, Clause 4, is granted to Congress as the power to establish “an uniform Rule of Naturalization.” This has long been interpreted by the courts as removing from the states the authority to deal with naturalization at all and as leaving the matter solely to the federal government. “That the power of naturalization is exclusively in Congress does not seem to be, and certainly ought not to be, controverted,” is how one chief justice of the United States, John Marshall himself, put it in 1817 in deciding a case called Chirac v. Chirac.

No doubt Arizona is going to argue that what it’s doing doesn’t intrude on federal authority in respect of naturalization. It simply deals with what immigrants can do once they get to Arizona, or, for that matter, what Arizonans can do about immigrants in their state. The law, according to a summary by the Associated Press, “makes it a crime under state law to be in the country illegally by specifically requiring immigrants to have proof of their immigration status.” It targets, as the AP puts it, the hiring of illegal immigrants as day laborers “by prohibiting people from stopping a vehicle on a road to offer employment” and by “prohibiting a person from getting into a stopped vehicle on a street to be hired for work if it impedes traffic.”

The Sheriff of Maricopa County in Arizona, Joseph Arpaio, who has drawn a national reputation with an aggressive campaign to arrest what he calls illegal aliens, went on television to praise the new law as another tool. In his remarks he noted President Obama’s statement of concern about the law and said he didn’t know why. “This is Arizona,” he said. “The people here, 78% of Arizonans want this law.” But the president has said his administration is going to be looking at the law. No doubt it is going to look at whether Arizona has over-stepped its bounds under the naturalization clause.

Also the 14^th Amendment. People often ask why illegal immigrants should have any rights at all under our constitution. The answer is found in the Constitution itself, which, in the 14^th amendment, states bluntly that no state may deprive “any person within its jurisdiction” — not just citizens but “any person within its jurisdiction” — the equal protection of the laws. The fact is that once a person is in America, on any terms, that person is protected, which is, no doubt, one of the reasons so many want to come here and one of the reasons why America has emerged as the shining city on a hill of which President Reagan, the grandson of what would be called today an illegal immigrant, spoke so eloquently.

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The big question today is whether the president and the congress are going to use the authority that the Founders enumerated and granted to them. Arizona’s new law is, in the broad sense, a challenge for them to do so, and it may even be that goading the federal government into action at the border was one of the animating factors behind the new law. We are in a period in which the federal government is asserting power the power to do things — regulate guns, issue paper money, require people to buy health insurance, to name but a few — that the Constitution fails to enumerate. To make a uniform rule in respect of naturalization, however, is one thing where the grant of power is unambiguous. The message Arizona is sending to the president is, in effect, use it or lose it. It’s hard to imagine that Mr. Obama wants to be the president who lost this power at a time when his party also controlled the Congress.