Imagine you are a legal immigrant to the United States. You have worked hard, kept a spotless criminal record, mastered English, and learned the rudiments of American government. On your 40th birthday, you go to the federal courthouse in Phoenix to be sworn in.

When the ceremony ends, you ask whether you can do what every citizen can do: register to vote. A helpful clerk hands you the National Mail Voter Registration Form, a postcard. Will this work in Arizona? Not to worry, the clerk assures you. Under the National Voter Registration Act (NVRA), states must "accept and use" this form to register voters. Fill it out, drop it in the mail, welcome to our political community.

On the form, you must attest on penalty of perjury that you are a U.S. citizen. The "state-specific" parts of the form tell you that you must also include your Arizona drivers' license number. You've been driving legally for years, since before 1996.

You mail the form, and wait.

Soon comes a letter from your county registrar. "Let this letter serve as notification that we have not yet received documentation of citizenship. ... Be advised that you are not a registered voter."

Under Arizona law, you learn, you must prove you are a citizen -- even though the federal form does not say so anywhere. Your driver's license number is not good enough -- because, unbeknownst to you or to the public generally, the state driver records have flagged your license with an F -- meaning "foreigner." The F isn't on your license; the general public doesn't even know F flags exist. (The "F" flag stopped being used in 1996, so most of those 33 and under don't face this problem.)

The Arizona law -- Proposition 200 -- sets out the acceptable documents. The law says you can us the number of your naturalization certificate, which of course you have right to hand. You fill it in and send off the new form.

Soon comes another letter. "Let this letter serve as notification that ..." Well, naturalization certificate numbers, it turns out, aren't stored in a database where county officials can use them, so despite what the law says, they aren't accepted. You can't mail in a copy, because it's illegal to photocopy one. The only way to register, it turns out, is to go to the courthouse and present the certificate in person. Nowhere on your federal form was there a warning that your driver's license and naturalization certificate numbers would be rejected.

Here's a question: Did state officials "accept and use" the federal form? Of course, says the state of Arizona. They allowed you to mail it in -- acceptance -- and read it so they could reject your application -- use. After all, they say, employers "accept and use" job applications; they just reject most of them.

No, say a group of Arizona voters and voter advocacy organizations; it is the clear intent of the NVRA that a completed form should provide all the information the state needs. It can check the information against public records to determine citizenship. It can send a letter announcing your registration, and then suspend if the letter is returned as undeliverable. What it can't do is make the voter jump through more hoops than the form requires.

At oral argument Monday in the U.S. Supreme Court, it seemed as if the result might turn on fine parsing of the words "accept and use." But lurking behind the statutory argument was another, more unusual one: at least four of the justices made it clear that they thought they could design a better form.

The textual power at issue in Monday's case, Arizona v. Arizona Inter-Tribal Commission, is granted in Article I § 4 of the Constitution:

The Times, Places and Manner of holding Elections for Senators and Representatives, shall be prescribed in each State by the Legislature thereof; but the Congress may at any time by Law make or alter such Regulations, except as to the Place of Choosing Senators.

The NVRA, or "motor-voter" bill, is an exercise of the "make or alter" power. In 1993, Congress determined that many states made it too hard to register. They empowered a federal agency to design a form -- and said that the form "may only" ask about certain things, such as age, address, and citizenship. Then they said all states are required to "use and accept the form."

Flash forward to 2004. Voters in Arizona became concerned that illegal aliens were flocking to the polls. They enacted Proposition 200, which requires the state to reject any form not accompanied by "satisfactory evidence of citizenship" -- a class of documents rather whimsically defined, as the naturalization-certificate-number snafu illustrates.

After Prop 20, 31,000 eligible voters -- citizens all, and most native-born -- were rejected for registration for failing to satisfy its demands. Only about 11,000 were eventually able to vote. The rejected voters were mostly native-born citizens; only about 20 percent were Latino. There was no racial or party identification to this cohort.

Meanwhile, community voter-registration drives all but collapsed. Set these exclusions against the evidence in the trial court that in 2005, only ten non-citizens (who would have been registered before Prop 200 took effect) even tried to vote, and only four succeeded. In 2001, Arizona's own Election Director had pointed out that very few aliens even try to vote, because if they are caught they will be deported. "Those who are in the country illegally are especially fearful of registering their names and addresses with a government agency for fear of detection and deportation," she wrote.

At the Supreme Court, however, the issue of the qualified voters excluded by Prop 200 took a distinct back seat to contempt for the NVRA and the form it produced. "It seems to me the federal form, as some of my colleagues have indicated, is not worth very much," Justice Anthony Kennedy told Arizona Attorney General Thomas Horne.

Justice Scalia later suggested that Arizona should have challenged the form, which required only a statement under oath that the registrant is a citizen. "Big deal," he said. "If ... you're willing to violate the voting laws, I suppose you're willing to violate the perjury laws."

Later, Chief Justice John Roberts quizzed Patricia Millett, representing the challengers, "You've got a great deal of reliance on what [the federal agency] has done, and I'm saying if it's not doing a very good job. ... I'd question whether or not the fact that [the agency] is going to implement it is sufficient assurance that the Act reads the way you say it does."

Justice Samuel A. Alito objected to the idea that, as the statute now reads, a state might require more information on its own forms but would be required to register voters using the federal form as well: "This seems to me like a crazy system."

Millett tried twice to remind the Court that this was a case about the right of individuals to vote in federal elections, not about the rights of the states. The argument for preemption of Prop 200, she said, "is in those 31,550 people who couldn't register." But she made little headway. By argument's end, the conservative bloc seemed lined up solidly behind Arizona.

Perhaps the challengers' best hope lies in an administrative-law argument advanced by Scalia, a former administrative specialist. Why, he asked repeatedly, did Arizona not challenge the agency's decision in a proceeding under the Administrative Procedure Act? Horne responded that he didn't know: "[T]hat was under a predecessor of mine." One or two Justices might support dismissing this case on those procedural grounds.

The state of Arizona, this Mississippi of the 21st century, has been before the Court repeatedly in the past few years. Perhaps that's where it picked up the idea that nobody needs to pay attention to Congress.