Marshfield — The Wisconsin man who blew the whistle on political campaigning by postal workers says U.S. Senator Tammy Baldwin’s office never responded to his concerns, an accusation Baldwin’s office denies.

Timm Kopp, a letter carrier from Marshfield, grew concerned that postal workers could take time off work to campaign for their unions’ preferred Democratic Party candidates, causing staff shortages.

A federal investigation found the U.S. Postal Service had violated a federal law, and the agency has promised changes.

“I never expected it to get to this point. Basically, all I did is, I wanted to raise concerns about things that were not being done correctly,” Kopp said during July 19 testimony before the U.S. Senate Homeland Security Committee.

Investigators found that the Postal Service allowed nearly 100 workers in several states, including Wisconsin, to take unpaid time off to do political activity for their union. The practice cost the Postal Service $90,000 in overtime to fill staff shortages, the U.S. Office of Inspector General found.

In his written testimony, Kopp said he called the offices of Baldwin, U.S. Sen. Ron Johnson, and U.S. Rep. Sean Duffy last fall. He wrote that it took about a week before Johnson’s office responded. Duffy’s office also called back, he said.

“I got no reply at all from Senator Baldwin’s office,” Kopp said in his testimony.

Bill Neidhardt, a spokesman for Baldwin’s office, said the senator’s staff had no record of Kopp contacting the office about the Postal Service ethics issue.

“We have records of Mr. Kopp contacting our office by email over 30 times between 2013 and 2015 about various other issues which he got a response to, but our office has no record of Mr. Kopp emailing us or connecting with our office about this issue in 2016,” Neidhardt said.

The Office of Inspector General told Baldwin’s office about the Postal Service issue at the end of November, after the 2016 election, Neidhardt said.

During a follow-up telephone interview with FOX6 News, Kopp was adamant he called both Wisconsin senators’ offices so it wouldn’t become a political issue. He said he kept no records of his phone calls.

During his Senate committee testimony, Kopp said he wasn’t interested in a political controversy.

“I did not want this to be a partisan thing,” he said July 19th. “I wanted the general public to not lose trust in the integrity of the Post Office.”

Wisconsin Republicans criticized Baldwin over the issue.

“This is yet more evidence that after two decades in Washington, Senator Baldwin puts her liberal special interest allies ahead of Wisconsin,” said Alec Zimmerman, a GOP spokesman.

Postmaster General Megan Brennan said her agency will stop allowing workers to take unpaid leave to campaign for political candidates.

Baldwin supports that, and favors Congressional oversight to ensure the changes happen, Neidhardt said.