What an illuminating exchange erupted at the Supreme Court this week between Justices Sotomayor and Scalia. It came during oral argument on Arizona’s requirement that persons seeking to register to vote produce evidence of citizenship, such as a driver’s license.

The National Voter Registration Act provides that in federal elections the states must “accept and use” the federal mail-in form, which contains only a sworn statement from the person that he or she is a citizen. It is unclear whether that means a state can never require additional information.

It was difficult to tell from the oral argument in Arizona v. Inter-Tribal Council of Arizona which side will prevail, since these days guessing the outcome of generally depends on divining how Justice Kennedy will vote, and in this case he asked incisive questions of each side. It is not too soon to declare that the winner in a sort-of side contest was Justice Scalia.

The Arizona case turns on whether the language of the NVRA preempts Arizona from asking for additional information, even if necessary to serve a compelling state interest such as preventing voter fraud. Congress did not say accept and use “under all circumstances,” which arguably leaves room for a state to add a requirement that would not be unduly burdensome and protects the integrity of the electoral process.

In such cases, it is common for courts to turn to legislative history — committee reports, floor debates, draft bills, etc. — to determine the intent of the legislature. Justice Scalia, though, has long argued that judges should disregard legislative history and decide cases based on what the legislature actually said (or didn’t say) in its statute. He argues that one can usually find whatever one wants in legislative history, and that it is only the final statutory language adopted by Congress that is determinative.

Justice Scalia, in his book “A Matter of Interpretation,” cites the oft-repeated statement that extracting something from extensive legislative history is like looking over the heads of a crowd to find a friend. There is something for everybody. His point was inadvertently demonstrated by Justice Sotomayor during oral argument in the Arizona case. She thought the legislative history clinched the position against Arizona, and she had the following colloquy with Arizona’s attorney general:

JUSTICE SOTOMAYOR: Some of us have — do believe in legislative history. Some of my colleagues don’t … But at least one of -­ (Laughter.)

JUSTICE SOTOMAYOR: Did he point to himself?

One of the concurring judges below said that he found the statute ambiguous, but that with the — the legislative history there just could be no conclusion but that Congress rejected your reading. Legislative history is very clear that this issue of what States could add to the form was raised and permission to do so was proposed explicitly and rejected. How do you—assuming that I believe in legislative history, don’t argue to me that I shouldn’t, okay?

MR. HORNE: Yes, Your Honor.

JUSTICE SOTOMAYOR: How do you get around that?

MR. HORNE: Your Honor, if the—if the legislative history were consistent, I would—I would say that was an argument that could be made. But the legislative history here is extremely self-contradictory and one cannot conclude from any part of that legislative history what was the intent of the majority of the Congress.

The House committee which dealt with the very act that—that we have said: “Only the elected officials designated and authorized under State law are charged with responsibility to enroll eligible voters on the list of voters. The NVRA should not be interpreted in any way to supplant that authority. The committee is particularly interested in ensuring that election officials continue to make determinations as to applicants’ eligibility such as citizenship as they’re made under current law and practice.” And the FEC, which is a predecessor to the EAC, relying on that House committee report, said that an application received by a local voter registration official is only an application and be subject to whatever verification procedures are currently applied to all applications.

In addition, Your Honor -­

JUSTICE SCALIA: Gee, if I believed — if I believed in legislative history, I would find that very persuasive.

(Laughter.)

The first instance of laughter was from the audience’s recognition that Justice Sotomayor was obviously referring to Justice Scalia, sitting three seats to her left. The second instance was a recognition that the colloquy had just demonstrated Justice Scalia’s point about the use of legislative history and the ability of each side to find something in it (but not something on which Congress had actually voted).

Justice Alito posed the following hypothetical to the lawyer challenging the Arizona law, seeking an admission from her that a state was not bound to accept the federal form under all circumstances:

A person rides up to a place to register on a bicycle and gets out and hands in the Federal form. This boy looks like he is 13 years old and he is carrying school books, he is wearing a middle school t-shirt, but he has filled out the form properly. Are they required to register him or can they ask him, could you show me some proof of age, like he would have to if he tried to buy alcohol or cigarettes?

The counsel answered that Arizona could not require anything from the 13-year old, but could apply “their own evidence that they obtain, whether it’s from their own eyeballs or … records or databases.” Arizona’s attorney general responded that it made no sense to say Arizona could use additional information, beyond what was on the federal form, but could not seek the evidence from the applicant himself. The federal statute contains no such distinction. The colloquy seemed to establish that “accept and use” cannot be construed as an absolute federal command, where Congress did not expressly make it one.

It is not clear how all the Justices will vote in this case. Several Justices (including Scalia) asked both sides questions that leave doubt on what the vote will be. But it is highly unlikely that Justice Scalia, whether he writes a majority opinion, concurring opinion, or dissent, will be relying on legislative history, and his joke during oral argument provided a good lesson on why he thinks it is improper to do so.

Mr. Richman is a contributing editor of The New York Sun.