Political apathy worried Jean-Jacques Rousseau. In a healthy republic, he wrote in “The Social Contract” in 1762, citizens “fly to the assemblies” and take an active role in public affairs. He would frown on America’s voter turnout, which hovers at 40% for mid-term elections and seldom goes much higher than 55% for a presidential race. But he might have been even more alarmed by laws that sideline infrequent voters from politics.

On January 10th a rule that has disenfranchised hundreds of thousands of Ohioans comes under the Supreme Court’s microscope. Husted v Philip Randolph Institute concerns what the League of Women Voters and the Brennan Centre for Justice calls the most restrictive approach to winnowing voter rolls found anywhere in America. Since 1994, in addition to nixing people who have died or moved—which all states do—Ohio has sent a postcard to voters who have not voted for two years. If they fail to return the address confirmation and then miss two more federal elections, they are taken off the rolls.

Ohio says its “supplemental process” for trimming voter lists is authorised by two laws: the National Voting Rights Act (NVRA) of 1993, which demands “accurate and current voter registration rolls”, and the Help America Vote Act (HAVA) of 2002. Challengers reply that Ohio’s procedure clashes with the primary mission of the NVRA: protecting the “fundamental” right of American citizens to vote. When the Sixth Circuit Court of Appeals considered Husted in 2016, it sided with the plaintiffs by a 2-1 vote. Inactivity, the majority found, is an illicit basis for wiping registered voters from the rolls. According to the NVRA, no individual may be removed from state lists “by reason of the person’s failure to vote”. In their brief to the justices, lawyers for the American Civil Liberties Union explained why Congress insisted on this caveat: “While voting is a right, people have an equal right not to vote,” a Senate report declared, “for whatever reason.”

The core of Husted involves a tricky detail of statutory interpretation: how to read a line in HAVA barring states from removing individuals from voter lists “solely by reason of a failure to vote”. The challengers and the Sixth Circuit majority contend that Ohio de-registers voters based only on inactivity—and thus falls foul of the law. Ohio’s Republican secretary of state, Jon Husted, and the dissenting Sixth Circuit judge, counter that the erasure results from that fact and another omission: the individual’s failure to confirm his address by sending the postcard.

For decades, the Department of Justice (DOJ) understood the law the way the challengers do. But in a rare about-face the DOJ under Donald Trump now embraces Ohio’s position. Samuel Bagenstos, a law professor at the University of Michigan who served in Barack Obama’s DOJ, laments, in an amicus brief drafted for 13 former DoJ lawyers who served in Democratic and Republican administrations, that the department’s “longstanding view” has been “abandoned”.

Mr Bagenstos’s brief examines the legal bases for voter purges. If Ohio wants to remove people who have moved, the brief notes, it is a better idea to consult post-office or motor-vehicle records than to guess from voting history. People may opt to stay at home on election day because they dislike the candidates, feel their vote won’t make a difference or are dissuaded by voter-ID laws: inactivity is not a reliable “indication that someone has moved”.

A study by Reuters found that about twice as many Democrats as Republicans were purged from the rolls in Ohio before the election of 2016. This partisan context may presage an ideologically split Supreme Court ruling, but Rick Hasen, an expert in election law, thinks Mr Bagenstos’s brief has a chance of swaying the court’s newest conservative, Neil Gorsuch. “I think if Justice Gorsuch is true to his textualist proclamations, his vote could well be in play,” Mr Hasen says. Elizabeth Wydra of the Constitutional Accountability Centre agrees. Justice Gorsuch insists that courts “must not rewrite federal laws under the guise of interpreting them”, she says, which bodes well for the plaintiffs. “States may not disenfranchise voters simply because they have failed to exercise their right to vote in the past.”