The United States Postal Service broke federal law by allowing its employees to participate in union-funded work to help elect Hillary Clinton and other Democratic candidates while on "union official" leave without pay and demonstrated an "institutional bias" toward union-backed candidates, according to an investigation.

The Office of Special Counsel determined USPS engaged in a "systemic violation" of the Hatch Act, which places limitations on the political activities of federal employees dating back to the 1990s.

Though federal workers are permitted to do political work while on leave from their agency, the Office of Special Counsel said the Postal Service demonstrated an "institutional bias" toward the candidates endorsed by the union, primarily Democrats.

"Specifically, USPS's practice of facilitating carrier releases for the union's political activity resulted in an institutional bias in favor of [National Association of Letter Carriers] endorsed political candidates, which the Hatch Act prohibits," the Office of Special Counsel found.

The investigation from the Office of Special Counsel began after Sen. Ron Johnson, R-Wis., who chairs the Senate Homeland Security and Government Affairs Committee, heard concerns from a constituent, Timm Kopp, about leave practices he experienced related to the 2016 election.

Kopp works as a letter carrier, and was concerned the Postal Service "incurred unnecessary overtime costs" and "improperly coordinated" with the NALC, the union.

According to the report, issued to the Senate Homeland Security and Government Affairs Committee on Wednesday, 97 members of the NALC requested leave without pay to participate in a program, called Labor 2016, to help elect Clinton and other union-backed candidates.

The Office of Special Counsel reported 82 percent of the work occurred in key battleground states, including Florida, Nevada, North Carolina, Ohio, Pennsylvania, and Wisconsin.

Those who assisted with Labor 2016 said they helped canvas, made phone calls for volunteers, and participated in other get-out-the-vote efforts.

Adam Miles, the acting special counsel for the Office of Special Counsel, said the NALC gave a list of letter carriers to participate in the campaign activity to a senior USPS labor relations official.

The official then emailed lists to lower-level USPS management officials nationwide.

According to Miles, who is testifying before the Government Affairs and Homeland Security Committee on Wednesday, the lower-level officials interpreted the messages from headquarters as "directives to release the carriers" on leave without pay.

The 97 union members who requested leave from the USPS without pay to participate in Labor 2016 were compensated through the Letter Carrier Political Fund, the union's political action committee.

"As a federal entity, the USPS must remain politically neutral," Miles said in his opening statement to the committee.

"In many localities, the Postal Service is a citizen's primary point of contact with the federal government, reinforcing the need for strict adherence to the letter and spirit of the Hatch Act," he continued.

USPS Postmaster General Megan Brennan attempted to distance the Postal Service from the union and said "senior postal leadership did not in any way guide union leadership in selecting the candidates for whom NALC employees should campaign."

"Our postal unions do not speak for the Postal Service, and the Postal Service does not speak for our unions," Brennan said in prepared testimony before the Homeland Security and Government Affairs Committee. "This especially applies in the political context, but it is inherent in any collective bargaining relationship."

Brennan also said any Hatch Act violations were unintentional, and committed the Postal Service to changing its practices based upon guidance from the Office of Special Counsel.