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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 ObamaObama to visit Kenya, South Africa for Obama Foundation in July Overnight Energy: EPA declines to write new rule for toxic spills | Senate blocks move to stop Obama water rule | EPA bought 'tactical' pants and polos Clarifying the power of federal agencies could offer Trump a lasting legacy 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) TesterDonald Trump Jr. headlines Montana Republican convention Overnight Defense: Trump orders Pentagon to help house immigrant families | Mattis says 'space force' needs legislation | VA pick gets hearing date Election Countdown: Family separation policy may haunt GOP in November | Why Republican candidates are bracing for surprises | House Dems rake in record May haul | 'Dumpster fire' ad goes viral MORE is the number one recipient of lobbyist campaign cash in the country."

Julian Pecquet contributed to the reporting of this article.