Montana congressman Denny Rehberg's bid to unseat Sen. Jon Tester Jonathan (Jon) TesterOVERNIGHT ENERGY: Court removes Pendley from role as public lands chief | Pendley court ruling could unravel Trump's public lands decisions | 1 in 4 adults cite climate change in decision not to have children Pendley court ruling could unravel Trump's public lands decisions Democrats shoot down talk of expanding Supreme Court MORE (D-Mont.) took a hit Sunday after the Associated Press uncovered lobbyist contributions to the Republican hopeful's campaign.

The AP reports that Rehberg's campaign received about $20,000 through October from three dozen lobbyists who didn't reveal their place of employment on federal disclosure records. The revelation could hurt Rehberg because he has relentlessly attacked Democratic first-termer Tester for being the top recipient of lobbyist campaign funding.

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The Tester campaign immediately began distributing the AP story to reporters.

“This is exactly why Montanans can't trust Dennis Rehberg,” Tester campaign spokesman Aaron Murphy said in a statement. “He just called for '100 percent transparency' in campaign fundraising, then he hid the money he takes from Washington lobbyists.”

The transparency comment refers to an interview Rehberg gave to the Flathead Beacon last month. “Every dime ought to be disclosed,” Rehberg told the newspaper.

The Rehberg campaign told the AP that the donors themselves are responsible for making sure the forms are complete.

"We not only comply with the law, we make every effort to go above and beyond it," said Chris Bond, a spokesman for Rep. Rehberg, in a statement to The Hill.

"This is all done in the light of day in publicly available documents and we believe that transparency is a good thing, because it allows folks in Montana to know that contrary to his rhetoric, Jon Tester is the number one recipient of lobbyist campaign cash in the country."

Montana is one of 23 Democratic-held seats up for grabs this November. A Tester win could shift control of the Senate to the GOP.

This story has been updated at 4:07 p.m.