Updated 10:41 a.m. | The conservative group Americans for Prosperity is launching a multi-million dollar ad campaign Monday aimed at two vulnerable Senate Democrats over their vote against a bill overhauling the tax code. 

Americans for Prosperity, which is backed by billionaires Charles and David Koch, is dedicating $4 million  for television and digital ads targeting Sens. Claire McCaskill of Missouri and Joe Donnelly of Indiana. Both senators are running for re-election in states that President Donald Trump won by wide margins in 2016. 

“Joe Donnelly and Claire McCaskill promised tax reform for years but chose partisan politics over Indiana and Missouri families when they had a once-in-a-generation opportunity to provide tax relief,” Americans for Prosperity President Tim Phillips said in a press release.

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No Senate Democrats voted for the GOP bill to overhaul the tax code. McCaskill wrote in a Medium post that the bill would not lead to a simpler tax code and would “give the vast majority of benefits to millionaires and billionaires, while working families get hurt.” Donnelly said in a statement he would vote against the bill because it was “a partisan tax hike on Indiana’s middle class” and would not curb outsourcing.

Americans for Prosperity highlighted McCaskill and Donnelly’s past statements calling for a tax overhaul, as well as announcements of employee bonuses in the wake of the GOP tax bill becoming law. 

“Sen. Joe Donnelly said he’d support tax cuts for hardworking Hoosiers. But when he had the chance, he said no, voting against tax cuts for you, standing with Chuck Schumer and Nancy Pelosi instead of us,” the narrator in the 30-second ad says. 

The video ads will run for three week on broadcast, cable, and satellite channels, as well as online.

Republicans are expected to tout the tax bill in House and Senate races across the country in 2018. Americans for Prosperity has announced it intends to spend $20 million highlighting the benefits of the tax legislation. 

Both McCaskill and Donnelly’s campaigns responded to the ads Tuesday morning, both saying they would have preferred to work across the aisle on a tax bill, but Republicans did not effectively engage with Democrats. 

“Joe tried time and again to work with Republicans to craft a tax bill that overwhelmingly helps working families and encourages companies to bring jobs back from foreign countries, but when Republicans crafted a purely partisan bill that does neither, Joe stood up for Indiana,” said Donnelly campaign spokesman Will Baskin-Gerwitz. 

McCaskill’s spokeswoman Meira Bernstein also said in a statement, “Claire’s record of working across the aisle to get results for Missourians speaks for itself — and no amount of money from the Koch Brothers is going to change Claire’s commitment to putting the people of Missouri before dark money billionaires and special interests like them.”