This law also placed limits on how states could remove voters from the rolls once they registered. OH is one of seven states, along with Georgia, Montana, Oklahoma, Oregon, Pennsylvania and West Virginia, that purge infrequent voters from registration lists, according to the plaintiffs who sued OH in 2016.
Sotomayor also said people have a "right not to vote".
After more than an hour's debate, neither side had a clear majority, largely because Justices Neil Gorsuch and Clarence Thomas - likely to be on Ohio's side - remained silent.
Conservative U.S. Supreme Court justices were joined by liberal Stephen Breyer on Wednesday in signaling sympathy toward Ohio's policy of purging infrequent voters from registration rolls - a practice critics say disenfranchises thousands of people - in a pivotal voting rights case. A ruling for OH could prompt other states to adopt the practice, which often pits Democrats against Republicans.
Sotomayor pressed Murphy on the consequences the OH process would have, particularly on poor and minority populations.
The case before the court centers on Republican-ruled OH where, under current law, state authorities can take away the right to vote from anyone who fails to show up at the polls for several years in a row.
OH said the court is required to resolve any ambiguity terms from the NVRA in favor of the state.
Partisan tweaks of election laws to either make it easier to vote or impose more exacting restrictions are a constant across the country.
January is shaping up to be a big month for Republican vote suppressors-and not in a good way for anyone who believes American politics benefit when more people vote.
In its July brief, OH argued that it removes voters from the rolls only when they do not respond to confirmation notices.
If a registered voter in OH does not vote in a two-year period, they are sent a notification to verify eligibility by a local election board, and removed from the rolls if they fail to respond and do not vote in the next four years.
At issue is a method OH uses to identify people who have moved and are no longer eligible to vote. OH says that it's not removing people because they didn't vote; failure to vote merely "triggers" the notice, and it's the failure to return the notice that results in people being removed from the rolls.
Registrants are targeted for removal from the voter rolls after failing to vote in one election and could ultimately be removed if they do not vote in the following four-year period. If a voter fails to return the notice, and does not vote over the next four years, the voter is removed from state rolls. Many of them arrived at the polls to vote only to learn that they were no longer registered.
The nonprofit groups challenging this method of voter pruning says OH is not alone: Georgia, Montana, Oklahoma, Oregon, Pennsylvania and West Virginia all use a similar process.
Solicitor General Noel Francisco said they concluded the law is poorly drawn, but there's no way for it to make sense unless states are given some flexibility to manage their lists.
While it's not unusual for department positions to change over time or with different administrations, flipping sides in the middle of a case is highly unusual.
Civil rights groups contend that a decision for OH would have widespread implications because it would fuel a broader effort to make it more hard and costly to vote.
A federal appeals court ruled for the state, concluding that roughly 7,500 OH voters - in a state that's perennially a presidential battleground - were wrongly purged from the list in the 2016 election.
Murphy again emphasized that the state's practice passes muster because "no one is removed exclusively by reason of their failure to vote". The evidence shows, Smith responded, that most people throw such notices in the trash.
The state learns nothing about whether someone actually has moved if a notice is not returned, he said.
In this context, there are several regulatory and technological innovations that are demonstrably superior to Ohio's supplemental process. He said he never saw the notice. "We are fighting in every state to protect and expand the right to vote".
Progressives say that next thing is voter purging, which is the focus of Ohio's case before the nation's highest court. In May, the Supreme Court agreed to hear the case.