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“We’ve done a lot of good things in the last five to six years,” he told reporters in Helena Wednesday, according to Helena's Independent Record. “We can’t let the negative ads running against us … make me something that I’m not.”

Rehberg, meanwhile has been hammering Tester as an ally of President Barack ObamaBarack Hussein ObamaMontana governor raises profile ahead of potential 2020 bid Trump was right to ditch UN’s plan for handling migrants Ex-White House stenographer: Trump is ‘lying to the American people’ MORE, who remains unpopular in the state, especially on the issue of healthcare reform.

But Rehberg has also had to grapple with reports that he received tens of thousands of dollars last year from lobbyists who did not properly disclose their place of employment. Rehberg has hammered Tester as a top recipient of lobbyist campaign funding.

"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 TesterJonathan (Jon) TesterHistory argues for Democratic Senate gains Overnight Defense: Trump inviting Putin to DC | Senate to vote Monday on VA pick | Graham open to US-Russia military coordination in Syria Senate to vote Monday on Trump's VA nominee MORE is the number one recipient of lobbyist campaign cash in the country."

Julian Pecquet contributed to the reporting of this article.