Senator Heidi Heitkamp, the centrist Democrat from North Dakota who is fighting a pitched battle to save her seat, has gotten a lift from an unlikely source: the conservative billionaire Koch brothers.

Americans for Prosperity, the political advocacy group backed by Charles G. and David H. Koch, unveiled a digital ad campaign on Friday thanking Ms. Heitkamp for her support of recently passed legislation that loosened regulations on small and medium-size banks that were swept up in the 2010 Dodd-Frank law passed in the wake of the 2008 financial crisis.

The ad is the latest evidence that the brothers, often at odds with President Trump, may be moving away from strictly partisan work on behalf of Republicans in favor of initiatives that will advance their libertarian-leaning legislative priorities, including free trade, immigration legislation and access to medicines, regardless of party affiliation.