A Democratic super PAC released an ad on Friday defending Sen. Joe Donnelly Joseph (Joe) Simon DonnellyTrump meets with potential Supreme Court pick Amy Coney Barrett at White House Names to watch as Trump picks Ginsburg replacement on Supreme Court Momentum growing among Republicans for Supreme Court vote before Election Day MORE's vote against the new tax law, after a group backed by GOP mega-donors Charles and David Koch rolled out an ad criticizing the Indiana Democrat on the issue.

The ad from Senate Majority PAC, which is aligned with Senate Minority Leader Charles Schumer Chuck SchumerJacobin editor: Primarying Schumer would force him to fight Trump's SCOTUS nominee CNN's Toobin: Democrats are 'wimps' who won't 'have the guts' to add Supreme Court seats Republican senator says plans to confirm justice before election 'completely consistent with the precedent' MORE (D-N.Y.), features an Indiana man named George Poynter, who argues that by opposing the new law, Donnelly is fighting for the middle class.

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"Out-of-state billionaires are attacking him for it, but I know Joe Donnelly is always looking out for Indiana," Poynter says in the ad.

Donnelly is one of 10 Democratic senators up for reelection in states that President Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE carried in 2016. Both parties are focusing on the new tax law in this year's midterm elections, with Republicans arguing that it gives middle-class families bigger paychecks and Democrats calling it a corporate giveaway and arguing that the law's impact on the national debt will lead to cuts in Medicare and Social Security.

Republicans are hoping to defeat Donnelly in an effort to expand their majority in the Senate. To that end, the Koch-backed Americans for Prosperity released an ad earlier this week attacking Donnelly over his stance on the tax law. The group is spending more than $2 million on the ad.

Americans for Prosperity on Friday also launched a campaign to thank lawmakers who supported the bill and asking voters to hold opponents accountable.

But Democrats are pushing back, and hope that Donnelly can win reelection and help them take back control of the Senate.

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“Joe Donnelly’s priority will always be protecting Medicare and Social Security for Hoosier families,” said Senate Majority PAC spokesman Chris Hayden said. “The Koch brothers have made Joe one of their top targets because, unlike his opponents, he does not put the craven desires of out-of-state billionaires first.”

In response to Senate Majority PAC's ad, Americans for Prosperity's Indiana State Director, Justin Stevens, said that “middle-class Hoosiers are seeing real benefits from the new tax law and won’t be talked into turning against it by a misleading ad from New York’s Chuck Schumer Chuck SchumerJacobin editor: Primarying Schumer would force him to fight Trump's SCOTUS nominee CNN's Toobin: Democrats are 'wimps' who won't 'have the guts' to add Supreme Court seats Republican senator says plans to confirm justice before election 'completely consistent with the precedent' MORE that essentially tells Hoosiers not to believe their bigger paychecks."

“The new tax law is working and gaining in popularity because of pro-growth policies that Sen. Donnelly said he supported but then voted against," Stevens added. "Sen. Donnelly’s ‘No’ vote gets harder to defend with each increased paycheck, bonus, and new job enjoyed by Hoosiers."

—Updated at 5:23 p.m.