The office of Democratic Sen. Tammy Baldwin (Wisc.) was the only office that failed to respond to complaints of postal workers illegally conducting political activity in Wisconsin during the 2016 campaign season for Democratic candidates, actions that left some postal offices in the state short on personnel, according to a whistleblower.

The union boss at the center of the incident in question is a longtime donor to Baldwin, records show.

Timm Kopp, a letter carrier for the U.S. Postal Service (USPS) in Marshfield, Wisc., and a member of the National Association of Letter Carriers (NALC), lodged complaints last year over postal workers who were taking approved time off from their jobs in order to campaign for candidates they believed would push for legislation that benefits the postal service.

During testimony before the Senate Committee on Homeland Security and Government Affairs, which is chaired by Wisconsin Republican Ron Johnson, Kopp spoke about his efforts to inform politicians from both parties of political campaigning by postal workers, activities he suspected were illegal.

Kopp testified that new postal workers must go through orientation that includes discussion of the Hatch Act, which bars federal employees from conducting political activity. Employees are also given brief follow up talks on the Act, especially during fall election periods, Kopp added.

Kopp met Scott Van Derven, the president of the Wisconsin State Association of Letter Carriers, in the fall of 2016 when he attended one of Kopp's meetings. Van Derven gave a brief speech about bills in Congress for which unions hoped to gain support, a practice that is regularly conducted to keep workers up-to-date on where issues stand in regards to legislation. During the meeting, Van Derven mentioned that volunteers were needed to help campaign efforts for the 2016 elections.

A few weeks after Van Derven's visit, a carrier in Kopp's office said he intended to volunteer with campaigning and would be gone for up to five weeks, which left the office scrambling to fill his position. Kopp spoke to his supervisor who said two people were also going to campaign from the nearby Wisconsin Rapids office, which was already shorthanded by two to three workers on most days.

Kopp's supervisor planned to deny his worker's request for time off to campaign. However, management in the company overruled the supervisor's decision and told him he had to let the worker campaign.

Kopp, who was acting supervisor for six months in late 2015 to early 2016, said that the Wisconsin Rapids office sent numerous emails for help to cover their employee shortfall. Carriers were also forced to work overtime on their days off to compensate for the lack of staff. The workers who left the postal system to campaign were being compensated with funds from the union's political action fund, he was told.

Kopp sought answers and contacted management in other offices.

"Their response was that there's always overtime, this is no big deal, and it is for the betterment of the company," he said.

Van Derven told Kopp that the individuals campaigning were cleared from the highest levels of management in the Postal Service.

"To me that statement, if true, showed this was illegal and morally this needed to be looked into," said Kopp.

He then contacted the offices of Rep. Sean Duffy (R.), Sen. Ron Johnson (R.), and Sen. Tammy Baldwin (D.) about his concerns.

"I decided to see if I could get a better answer how this could be legal and called both Senator Johnson's office and Senator Baldwin's office," said Kopp. "I stated my concerns and said that at the very least I would like some sort of reply to this and would not like this thrown aside."

"A few days later after no response I also called my district representative Sean Duffy's office and stated my concerns," he continued. "It took about a week and I finally got a reply from Senator Johnson's office that they were going to look into this matter for me and would keep in touch. Sean Duffy's office called as a follow up to see if I was getting help. I got no reply from Senator Baldwin's office."

Van Derven, a longtime Democratic donor in the state, has made 11 donations to Baldwin's campaign committees directly and through Act Blue, which describes itself as "the online clearinghouse for Democratic action."

Van Derven did not return a request for comment.

An investigation ultimately found that USPS had violated federal law in a number of states, including in Wisconsin, according to local reports.

More than 100 workers in numerous states were found to take unpaid time off to conduct political activity on behalf of their unions, which ended up costing the company $100,000 in overtime pay to compensate for the lack of staff, an inspector general found.

Baldwin's office claims they never received the complaint from Kopp and were first made aware of the situation by the Office of the Inspector General in November of last year.

"We have records of responding to Mr. Kopp after he contacted our office by e-mail over 30 times between 2013 and 2015 about various other issues," Baldwin's office told the Washington Free Beacon. "However, our office has no record of Mr. Kopp emailing us or connecting with our office about this issue in 2016, and as he has said, neither does he."

The office said the senator supports the OIG's findings and recommendations and that she also supports further Congressional oversight to ensure the USPS makes necessary changes.

Alec Zimmerman, spokesman for the Republican Party of Wisconsin, said that Baldwin is more concerned with protecting special interests than Wisconsin.

"After two decades in Washington, Senator Baldwin is more concerned with protecting her liberal special interest allies than she is about Wisconsin," Zimmerman told the Free Beacon. "Just like when she failed to act when veterans were in danger at the Tomah VA, Senator Baldwin chose to protect the Washington status quo by ignoring this whistleblower’s complaints that government officials broke federal law."

The Committee on Homeland Security and Government Affairs, which Kopp testified before, also has oversight of the Tomah VA, a scandal-plagued veterans affairs center in Wisconsin that was overprescribing narcotics to veterans. The abuses at the facility led to the death of a Marine. Baldwin quietly stepped away from the committee earlier this year, the Free Beacon previously reported.

Baldwin was the only member of Congress from Wisconsin to receive a report of the abuses at Tomah but did not take action until it became public more than four months later and the veteran had died.

The Democratic senator tapped Marc Elias, a partner at the D.C.-based Perkins Coie law firm who is considered the "go-to fixer" for Democrats, for help on crisis control following the scandal. Elias, who was Hillary Clinton's top campaign lawyer, was paid $90,000 for his services.