MADISON, Wis. — A years-long fight over voting rules in the swing state of Wisconsin sparked a criminal investigation this week after a mayor — wearing a hard hat and Department of Public Works jacket — carted off a ballot drop box the city clerk was about to make available to voters.

The move by Wausau Mayor Doug Diny came as officials around the battleground state wrestle with whether to use drop boxes after the state Supreme Court this summer gave them that ability. Officials in Milwaukee, Madison and other Democratic strongholds have embraced them, while their counterparts in some of Milwaukee’s suburbs and other Republican areas have banned them.

Nowhere has the dispute been more prominent than Wausau, a city of 40,000 people in the heart of the state. The mayor, who was elected in the spring with the help of the Republican Party, told the Wausau Pilot & Review that he moved the drop box to his office Sunday until he could address unspecified issues with the city clerk. He provided the publication with a photo of himself pushing the drop box on a handcart in front of city hall.

Clerks in Wisconsin have sent more than 410,000 absentee ballots since mail voting began last week, and voters can return them by mail, at clerk’s offices or to drop boxes.

The Wausau drop box was placed outside of city hall late last week, and the clerk planned to have it secured to the ground so that it could be used starting Monday, according to city officials. It was locked, and voters could not place ballots inside while it was awaiting installation, City Clerk Kaitlyn Bernarde said.

Bernarde said she discovered the drop box was missing Monday and reported the matter to Marathon County District Attorney Theresa Wetzsteon (D), who said Wednesday that she has launched an investigation. The drop box will not be used for now, but voters can return ballots in a separate box normally used for payments to the city government, Bernarde said.

Diny did not respond to requests for comment from The Washington Post on Wednesday. Critics this week called on the mayor to explain himself.

For years, drop boxes were uncontroversial and used by communities around the state. Their use exploded during the 2020 election because of the coronavirus pandemic, and more than 500 municipalities made them available that year.

In 2022, the state Supreme Court barred their use in a 4-3 decision because state law does not mention drop boxes. This summer, after liberals took control of the court from conservatives, the justices issued a 4-3 decision allowing drop boxes.

The latest decision left it to nearly 1,900 municipal clerks to decide whether to use them in their communities. Dozens of clerks, if not more, have decided not to use the drop boxes this fall, including all the clerks in Dodge County in eastern Wisconsin. Some of those clerks made their decisions after Sheriff Dale Schmidt (R) warned that drop boxes could “degrade trust in our system,” according to WisPolitics.com.

On Tuesday, a few Wausau residents expressed frustration with their mayor during a city council meeting — including one who noted former mayor Katie Rosenberg’s support for drop boxes. Ahead of a spring 2022 election, Rosenberg appeared in a dancing video with other Democrats to promote Wausau’s drop box.

“I just feel that this is further undermining election integrity and while we had a former mayor who promoted accessible voting, we have this one who would rather try to swipe a ballot box on a Sunday after church,” Nancy Stencil told the council.

Sarah Brock, a Wausau school board member who spoke during the council’s public comments period, said the mayor’s actions risked creating confusion.

“Our votes are our voice,” she said in an interview. “And if it feels like that is being silenced in any way or disrespected, that’s a really personal feeling.”

In brief remarks at the meeting, Diny said he hadn’t spoken with the city clerk about the drop box and claimed his behavior had been mischaracterized.

He told the Wausau Daily Herald that he moved the drop box inside city hall because it wasn’t secure and compared his activities to that of a maintenance worker. He told the Wausau Pilot & Review that people don’t like drop boxes and have many other ways to cast ballots. And he told a Wausau radio station that he put the drop box in a safe place “because what if it had been taken and thrown in the river?”

The city clerk on Monday found security footage of the mayor carting away the drop box, according to Lisa Rasmussen, the city council president.

Rasmussen said she emailed the mayor about the incident when she learned of it, and he told her he had put it in a secure location over unspecified concerns. He emailed her a copy of the photo of him removing the drop box, she said.

“Whether local elected officials agree with the use of drop boxes or not, it is not our decision to make and we have no authority to manage or tamper with drop boxes,” Rasmussen said by email. “It is the city clerk’s choice and she made it.”

The district attorney said she has asked for assistance with her investigation from the state Department of Justice. A spokeswoman for Wisconsin Attorney General Josh Kaul (D) said the department has not yet determined whether it would help.

Sam Liebert, the Wisconsin director of the voting rights group All Voting Is Local, said an investigation is essential.

“This is an egregious and physical attack on drop boxes by an election denier who is restricting voter access to the ballot box,” Liebert said in a statement.