A group backed by GOP mega-donors Charles and David Koch is launching new ads focused on the Republican tax law in two states where vulnerable Democratic senators are seeking reelection.

Americans for Prosperity (AFP) said it is spending nearly $4 million on digital and television ads in Missouri and Indiana, two states that President Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE won in 2016 and where Democratic Sens. Claire McCaskill Claire Conner McCaskillMomentum growing among Republicans for Supreme Court vote before Election Day Democratic-linked group runs ads in Kansas GOP Senate primary Trump mocked for low attendance at rally MORE and Joe Donnelly Joseph (Joe) Simon DonnellyNames to watch as Trump picks Ginsburg replacement on Supreme Court Momentum growing among Republicans for Supreme Court vote before Election Day Barrett seen as a front-runner for Trump Supreme Court pick MORE, respectively, are facing reelection.

The 30-second ads against Donnelly and McCaskill feature residents of their states talking about how the tax law benefits them and how they were disappointed by the senators voting against the measure.

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The ads, which will run for three weeks starting Thursday, are the second wave of spots AFP has released on the tax law in Missouri and Indiana. Trump carried both states by double digits in 2016 and the nonpartisan Cook Political Report rates both Senate races as toss-ups.

The Koch network has committed to spending up to $20 million to promote the tax bill signed into law by Trump in December. Republicans are hoping that highlighting the tax law and the strong economy will help them in the midterm elections, which are typically challenging for the party that controls the White House.

“As more Americans see the benefits of tax reform, they are rightly concerned to learn that some lawmakers, like Joe Donnelly and Claire McCaskill, voted against tax relief for individuals and families," AFP president Tim Phillips said in a statement. "Where Senator McCaskill sees ‘scraps,’ a working mom sees more money for groceries. Meanwhile, Senator Donnelly chose partisan politics over the opportunity to help workers and families in his state."

Donnelly campaign spokesman Will Baskin-Gerwitz said that the tax law will end up hurting middle-class Indiana residents.

"Joe tried hard to work with Republicans to craft a tax plan that helps middle class Hoosiers and doesn't cause skyrocketing deficits, but instead the McConnell tax plan will add as much as $2.2 trillion to the national debt over ten years -- billionaires like the Kochs get massive new tax breaks, while older Hoosiers get stuck with massive cuts to Medicare, Medicaid and Social Security to pay for it," he said.

"What's more, the McConnell tax plan gives billions of dollars in new tax breaks to the same health care companies who are set to wipe out any gains from the plan for middle class Hoosiers when they raise health care premiums by double digits," Baskin-Gerwitz added.

McCaskill camapign spokeswoman Meira Bernstein defended the Missouri senator's opposition to the tax law.

"This tax bill was a windfall for corporations and big drug companies. These companies have given billions of dollars to their shareholders while hiking the costs of prescription drugs for Missouri families. It's disgraceful," she said.

— updated at 5:06 p.m.