Can the state of Arizona require people to prove they are citizens when they register to vote? That’s the question the Supreme Court considered on Monday, when it heard oral arguments in a lawsuit challenging Arizona’s Proposition 200, a ballot measure adopted in 2004.

Arizona argues that the law guarantees the integrity of the electoral process. The groups challenging law, including the Obama administration, argue that Arizona’s law disenfranchises large groups of people, such as newly naturalized citizens, who will find it difficult to comply with the state’s requirements.

Remarkably, the legal questions in this case have almost nothing to do with the substantive question that really divides the litigants.

Everyone agrees that Arizona has the right to make U.S. citizenship a requirement for voting. Although some states in the 19th century allowed residents intending to become citizens to vote, because they wanted to attract immigrants, now citizenship is a requirement for voting everywhere in the United States.

Everyone also agrees that the states have the right and the duty to protect the integrity of our elections by stopping noncitizens from voting.

The two big questions here are about process: What evidence does a would-be voter need to provide in order to get on the voter rolls? And, given the current state of the law, who gets to decide what evidence will be enough?

In 1993, Congress passed the “motor voter” law, upon which Bill Clinton had campaigned. That law creates a single, national voter registration form, which all the states are required to “accept and use.” On that form, registrants simply have to declare that they are U.S. citizens, without supplying any supporting evidence beyond their word, though the law specifies that those falsely claiming citizenship may be prosecuted for perjury.

Arizona’s Proposition 200, however, says in effect that a voter’s word is not sufficient proof of citizenship. It requires voter registrants to supply additional evidence, such as a driver’s license number, that would enable the state to confirm each applicant’s citizenship.

The elections clause of the Constitution clearly gives Congress the right to regulate the conduct of federal elections; no one is challenging the constitutionality of the motor voter law. It’s just that the law doesn’t speak very clearly to the specific issue Arizona raises.

In fact, the motor voter law allows the states to ask for some state-specific information, in addition to what the federal government requires. It also, however, forbids the states from requiring other things, such as the form to be notarized.

Arizona argues, reasonably enough, that because the motor voter law expressly forbids states from imposing some, specified requirements, it implicitly permits other additional requirements. If Congress wanted to forbid states from asking for clear evidence of citizenship, it could have done so — but it didn’t.

Opponents of Proposition 200, however, argue that when Congress directed the states to “accept and use” the motor voter form, it must have meant for the states to accept as true the information on the form and not to demand additional evidence. What use is Arizona making of the federal form, they ask, if that form is not going to count as sufficient evidence of citizenship?

Besides, one major point of the motor voter law was to simplify the process of voter registration. Arizona’s demand for additional information only complicates things. The state replies that the law also identifies preserving the integrity of the electoral process as a goal of the law, and, it argues, the additional information Arizona is asking for is not all that different from information that other states already are allowed to collect.

Arizona, apparently, had asked the executive director of the U.S. Election Assistance Commission, which produces the federal registration form, to include its request for information alongside the other state-specific requests for information. That official refused, however, thinking that the Arizona’s requirement might disenfranchise some voters. When Arizona asked the whole commission to review that ruling, the commissioners deadlocked and let the decision stand.

That one official’s decision may turn out to be the pivot on which this case turns: the court often finds reason to defer to the judgments made by officials charged with implementing federal law.

Whichever way the court ultimately decides, this controversy illustrates one of the most important lessons of political science — controlling the procedures by which laws are made and applied, and voted on, goes a long way toward dictating the outcome.

Joseph R. Reisert is associate professor of American constitutional law and chairman of the department of government at Colby College in Waterville.