Americans for Prosperity, the political group backed by billionaire businessmen Charles and David Koch, has launched its an ad attacking North Dakota Democratic Senator Heidi Heitkamp for her vote against tax reform.

The $450,000 ad buy is the latest in a multi-million dollar effort against Democrats running for re-election in states that President Donald Trump won in 2016. 

The narrator in the ad says, "When Heidi had the chance to help us with real tax cuts, she turned her back - voted 'no."

The ad comes on the heels of $8 million worth of advertisements in Indiana and Missouri and is part of the $400 million the Koch network announced they'd spend on politics and policy tin 2018. 

A spokeswoman for Heitkamp's campaign, Julia Krieger, fired back on the ad, saying, "it's unfortunate when campaigns are reduced to flat-out lies from out-of-state billionaires. But North Dakotans know the truth about Heidi: She would never vote for a bill that could jeopardize Social Security and Medicare, or sacrifice the permanent tax cuts working North Dakotans deserve."