The United States Postal Service took heat during a congressional hearing Wednesday for allegedly violating federal law by “favoring” a union doing pro-Clinton campaign work.

According to a report released Wednesday morning by the Office of Special Counsel, USPS engaged in “systemic violations” of the Hatch Act, a federal law that limits certain political activities of federal employees.

While employees are allowed to do some political work on leave, the report said the Postal Service showed a “bias” in favor of the National Association of Letter Carriers (NALC) campaign operation, letting employees take leave without pay to participate in pro-Clinton campaign efforts.

The Senate Homeland Security and Governmental Affairs Committee heard testimony from OSC and USPS officials on Wednesday, along with the USPS mail carrier who shared complaints with Committee Chairman Ron Johnson, R-Wis., in October 2016.

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“’Postal Service leadership ‘took official actions with the intent of enabling’ the campaign activity of its union, and ‘with a clear understanding of what that activity involved,’” Senate Homeland Security Chairman Ron Johnson, R-Wis., said in his prepared opening statement, citing the OSC report.

“Based on these findings, it is legitimate to wonder why no one will be held accountable, how Postal Service leadership allowed this systemic violation of the Hatch Act to go on for twenty years, and is this occurring in other federal agencies?”

Johnson was first notified of the “violations” in October 2016 by Tim Kopp, a letter carrier for the USPS and member of the NALC. Kopp thought the actions were “illegal” and was concerned the Postal Service “incurred unnecessary overtime costs” and “improperly coordinated” with NALC when it released several members for weeks of “union official” leave without pay to participate in partisan campaign work.

“I want things done fairly and I don’t want things done on a partisan issue—the union is always involved in highly political activities and I didn’t want this to be a partisan thing—I wanted this to be a thing where the general public does not lose trust in the postal service,” Kopp told the Senate panel Wednesday. “There are a lot of good employees, but the way this was done—it was a nightmare.”

According to the OSC report, roughly 97 NALC members requested the leave without pay to participate in get-out-the vote efforts in primarily 2016 battleground states: Florida, Nevada, North Carolina, Ohio, Pennsylvania, and Wisconsin. The NALC, which endorsed Clinton last June, compensated those USPS workers using the Letter Carrier Political Fund, the union’s PAC.

Officials at multiple levels apparently were involved.

According to OSC Acting Special Counsel Adam Miles, the NALC provided lists of letter carriers to participate in campaign activity to a senior headquarters USPS labor relations official, who then emailed the lists to other USPS officials across the country. According to Miles, the local officials “interpreted the communications as directives” from USPS headquarters to release the carriers on union official leave without pay.

“The practice also put non-union employees, or union employees who supported other candidates, at a disadvantage,” Johnson said. “If those employees sought unpaid leave for several weeks for campaign activity, they would not have received the same treatment.”

The OSC found an “institutional bias” in favor of union-endorsed candidates, all of which were Democrats.

“USPS, through its longstanding practice of honoring these kinds of requests, failed to administer its programs in a politically neutral manner in violation of the Hatch Act,” The OSC report said.

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Committee Ranking Member Claire McCaskill, D-Mo., said that the USPS is “emblematic of a systemic problem not limited to one individual or one election cycle.”

“USPS has failed to implement sufficient controls to ensure compliance with the Hatch Act,” McCaskill said.

But USPS Postmaster General Megan Brennan told lawmakers that “senior postal leadership did not in any way guide union leadership in selecting the candidates for whom NALC employees could campaign” and that USPS “did not approve or choose candidates for the unions to support” or “ask the union to advocate for political candidates on behalf of the Postal Service.”

“I also note that our postal unions do not speak for the Postal Service, and the Postal Service does not speak for our unions,” Brennan wrote in her prepared testimony, insisting USPS did not seek to assist the NALC’s “favored candidates.”

Brennan said the practice to grant leave without pay for NALC political activity had been in place for approximately 20 years, but that all violations of the Hatch Act were “unintentional.”

“We will change our practice in consultation with the OSC and based upon OSC’s guidance. This will ensure that we do not put our people in harm’s way and they do not unintentionally run afoul of the Hatch Act,” Brennan said.

“As we have previously communicated to both this committee and to the OSC, and as the OSC has acknowledged, the Postal Service has always been ready, willing and able to end or modify our practice as appropriate, consistent with OSC’s recommendation.”