COMPARED WITH WHAT some Americans have to tolerate on Election Day, registering to vote is relatively painless. That’s partly thanks to the National Voter Registration Act, a 1993 law at the root of a case the Supreme Court will hear on Monday. The state of Arizona argues that it should be allowed to subvert the law’s obvious purpose. The court shouldn’t let it.

In 1993, Congress looked at the “complicated maze” of often confusing and sometimes discriminatory state election rules, and it found that “unfair registration laws and procedures can have a direct and damaging effect on voter participation in elections for federal office.” So lawmakers established national standards. Americans could register to vote when getting driver’s licenses, which gave the act its unofficial name: the “motor voter” law. Congress also required every state to accept a simple, common, mail-in registration form drafted by the U.S. Election Assistance Commission. The record indicates that Congress meant these to be among the “procedures that will increase the number of eligible citizens who register to vote in elections for federal office.”

In 2004, Arizona voters approved a state law requiring evidence of U.S. citizenship in order to register to vote. As a result, state elections officials no longer accepted standard federal registration forms unless accompanied by copies of passports, birth certificates or other proof of citizenship. Native American and Hispanic groups complained, and now the dispute is before the high court.

Arizona argues that federal law doesn’t bar the state from adding to the registration requirements listed on the federal form. It points out that Congress acknowledged maintaining the integrity of federal elections as another legitimate and important goal. Yet any reasonable reading of lawmakers’ intent requires the conclusion that they did not want to replace one confusing array of specific state requirements with another confusing array of specific state requirements. In fact, Congress rejected a provision that would have expressly permitted states to ask for evidence of citizenship during registration. On the legal merits, the justices shouldn’t have trouble siding against Arizona.

But what of the policy principle? Non-citizens shouldn’t be allowed to vote. But neither should citizens be discouraged from the exercise of their most essential right — even citizens who don’t have identification deemed suitable by Arizona. States should be instituting reforms to expand access to the franchise, not narrow it. Universal voter registration, in which the states take responsibility to register all who are eligible, would be a good start.