A federal judge who has previously upheld Maine’s ranked-choice voting law heard arguments Thursday that the law is unconstitutional because it may disenfranchise voters who are too old, confused or uneducated to understand how it works.

The lawsuit by four Maine voters – three Republicans and one independent – seeks to stop Maine’s the first-in-the-nation voting system from being used in the fall general election. The suit names Secretary of State Matt Dunlap, Gov. Janet Mills and Maine Attorney General Aaron Frey as defendants.

But Bangor District Court Judge Lance Walker has previously ruled that Maine’s law, first passed by voters in 2016, is constitutional. The law has withstood a barrage of legal challenges and was reaffirmed by voters again in a 2017 people’s veto vote that overturned a repeal of the law by the Legislature.

Walker heard testimony Thursday from one expert witness, a Princeton political science professor. He said his research suggests some Maine voters were being disenfranchised by the system because they did not rank candidates on their ballots, made only one choice in multi-candidate races or left their ballots entirely blank.

“There seems to be a substantial number of people who are not understanding the way (ranked-choice elections) work,” said Professor Nolan McCarty, who teaches politics and public affairs at Princeton University, during questioning by one of the plaintiffs’ attorneys, Stephen Obermeier.

The system allows – but does not require – voters to rank candidates in order of preference. If one candidate gets more than 50 percent in the first round, that person wins. If no one receives a majority, the ranked preferences are used to decide the winner.

The law withstood a federal legal challenge in 2018 after Democrat Jared Golden unseated Republican Bruce Poliquin in the 2nd Congressional District. Poliquin won the initial vote but did not have a clear majority. Walker later denied his request for a new election, and Poliquin ultimately dropped his legal challenge.

The plaintiffs in the latest lawsuit are four Maine voters: Robert Hagopian of Madison, Duane Lander of Greenville, Sterling Robinson of Warren and James Trudel of Hermon. Three are registered Republicans; one is a registered independent.

All four are in their 70s and have some level of college education, with three of the four holding advanced degrees either in business or engineering.

McCarty said his research suggests older voters are more likely to “truncate” or not fully utilize a ranked-choice ballot, selecting only one candidate instead of ranking them in order of preference. He said voters that had less than a college education were less likely to participate in a ranked-choice election because they didn’t understand how it works.

But Thomas Knowlton, an assistant attorney general for the state representing Dunlap, Mills and Frey, countered that McCarty really had no idea what motivated voter behavior. He was also dismissive of McCarty’s statistical analysis, which drew largely on ranked-choice elections in municipalities in other parts of the country that use similar ranking systems.

“The most that can be shown from his regression analysis is, it’s possible older voters truncate their ballots more often,” Knowlton told Walker during questioning following McCarty’s testimony. “Who knows why. Maybe they are more committed to the major party candidates, that’s not a constitutional violation. He has no idea why people voted the way they voted.”

But Obermeir countered that McCarty’s analysis showed, “a significant amount of voters did not make a choice, they were confused by the system.”

Walker, who said he would likely issue a ruling by the middle of next week, asked Obermeier what he meant when he referred to a “substantial number of voters.”

“I don’t know what that means,” Walker said. He said in traditional plurality elections, where the candidate with the most votes win, there were also systemic complications and failures. “We don’t necessarily conclude a priori they are unconstitutional,” he said.