In the divisive issue of voting access, there’s a rare area of consensus on the value of maintaining up-to-date voter rolls. Advocates on every side agree that it’s important to strike ineligible voters, both because it speeds up the process of voting and it reduces the possibility for voter fraud. But how to keep the rolls updated is highly contested — so much so that the Supreme Court is hearing arguments today on whether Ohio is using an illegal approach to strike voters from its lists.

At issue in the case, Husted v. A. Philip Randolph Institute, is whether the state’s procedure for maintaining accurate voter rolls, which uses failure to vote as a trigger for the removal process, is acceptable — even if legitimate voters are getting scrubbed in the process.

A review of Ohio voter data shows that the degree of purging varies from county to county, and in some cases the procedure’s application can depend on how local officials choose to implement it. Although these variations won’t be addressed before the court today, the court challenge highlights the extent to which an individual voter’s ability to cast a ballot could depend on where he or she lives, an increasingly pressing concern as states take divergent approaches to establishing voter eligibility.

Processes like Ohio’s may also introduce more opportunities for racial or political disparities in who gets purged from voter rolls. Critics of the Ohio approach, for example, contend that the procedure disproportionately affects minority voters affiliated with the Democratic Party, who tend to cast their ballots less frequently and are therefore at greater risk of being removed.

The decision in this case could easily have consequences for presidential elections, given Ohio’s importance as a swing state. Moreover, according to legal experts, the outcome could signal how the Supreme Court might rule in other, further-reaching cases on laws affecting the voting process.

In every state, local election officials have to engage in a bit of routinized detective work to maintain the accuracy of voter rolls: Officials compare their lists with change-of-address databases from the U.S. Postal Service, state and federal vital statistics that list recent deaths, and records of people who have been convicted of felonies. They then prune voters who have moved, died or lost their eligibility to vote.

It’s a fraught and complicated process, which is why a handful of states — including Ohio — use an additional measure in a bid for efficiency. Ohio’s local election boards first send notices to voters who haven’t cast ballots in the previous two years. If those voters don’t confirm their address and don’t vote in the subsequent four years or interact with the electoral system in other ways, like by signing a petition, they’re removed from the rolls — even if they’re haven’t moved and are still eligible to vote.

In the summer of 2015, Ohio used this process to remove tens of thousands of voters who hadn’t cast a ballot since 2008 — a potentially pivotal move in a state where recent presidential elections have been decided by as few as 100,000 votes.

Attorneys from Demos, a left-leaning think tank, and other voting rights groups sued in response to the purge, arguing that by using failure to vote as a trigger for sending the notice, Ohio had violated the 1993 National Voter Registration Act, which prohibits states from yanking voters’ registration merely because they didn’t participate in an election.

State officials, by contrast, say the removal happens because the voters didn’t respond to the notice, not because they didn’t vote. In their view, the system is protecting the integrity of the voter rolls by catching ineligible voters who haven’t notified the postal service that they’ve changed their address.

Ohio’s Republican secretary of state, Jon Husted, has pointed out that the process has been used under Democratic and Republican administrations. “Our goal is to make it easy to vote and hard to cheat,” said Matthew McClellan, the communications director for Husted’s office. “It benefits the voter to have accurate lists — there’s less processing time when they come to vote, and there are fewer opportunities for fraud.”

A federal appellate court did not agree. In response to the lawsuit, the U.S. Court of Appeals for the 6th Circuit struck down Ohio’s process less than two months before Election Day. Voters who had been slated to be purged were able to cast a ballot that November, but the fate of the process will now be decided by the Supreme Court.

Whatever the ultimate ruling, it’s clear that some of Ohio’s 88 local boards of elections purge voters for inactivity at much higher rates than others.

Some of these differences can be chalked up to local quirks. Linda Stutz — the director of elections for Van Wert County, which has one of the highest rates of purging due to inactivity in the state — said she suspected her county stood out because it’s just over the border from several large hospitals in Fort Wayne, Indiana.

“A lot of people go to those hospitals, and if they die, they die in Indiana,” she said. “That means we don’t get the same notification that they’re deceased, so it’s likelier that we won’t remove them immediately and they’ll go on inactive status when they don’t vote or respond to the notice.”

Karla Herron, meanwhile, is the director of the board of elections for Delaware County, which has one of the lowest rates of removal due to inactivity. She speculated that residents of her county — who are twice as likely as Ohio residents overall to have a bachelor’s degree — are more highly educated and therefore more politically engaged.

In several counties, though, rates of removal appear to be tied to variations in the way the process is applied. Herron said that she and her team painstakingly double-check voters’ status before removing them; others rely on computer systems to tell them if a voter has been mistakenly labeled inactive; and one elections official admitted that she doesn’t always remove voters from the rolls when she’s supposed to.

Only 122 of Wyandot County’s 15,369 registered voters as of November 2014 were removed for inactivity in 2016. The director of the board of elections, Debra Passet, stressed that this was partially due to her team’s reluctance to remove voters for inactivity. “We really try to give people the benefit of the doubt,” she said. “I’ll leave people on the rolls for a few extra years, because what’s the harm in keeping them on there?”

Stuart Naifeh, the Demos attorney arguing the case before the Supreme Court, said he wasn’t surprised to hear that some counties were more zealous about purging voters than others. For a while, he said, counties were sending out different types of notices to warn voters that they were about to be placed on inactive status. “Some counties were sending a postcard, which is easier for people to miss or ignore than a letter in an envelope,” he said. “But it was also simpler to deal with than what other counties were sending out, which was a more detailed notice that essentially required people to reregister to vote.”

For critics, the lack of uniformity highlights just how problematic the process can be. “In theory, it’s supposed to be uniform, but in practice it can be quite individualized,” said Freda Levenson, legal director of the ACLU of Ohio, one of the other organizations challenging the state’s process. “We really don’t want a person’s ability to vote to be determined by the luck of where they live.”

And if voters show up to vote and find out they’ve been scrubbed from the rolls, there can also be variations in the process for obtaining provisional ballots, which would be counted if the voters are determined to actually be registered.

Of course, Naifeh and the other attorneys on his team contend that even if the process were being implemented completely consistently across the state, it would still be illegal. That perspective is shared by some local elections officials, who signed onto an amicus brief arguing that there are better ways to maintain the voter rolls than Ohio’s process. They said that the possibility that legitimate voters may be scrubbed from the rolls wasn’t worth the risk.