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WASHINGTON — A Charles and David Koch-backed group is launching a $4.5 million ad campaign targeting three Democratic senators, urging them to support the Republican tax reform plan.

The television ad buy, announced Thursday morning, will target Sens. Joe Donnelly of Indiana, Claire McCaskill of Missouri and Tammy Baldwin of Wisconsin, all of whom are facing tough re-election efforts next year in states that President Donald Trump won.

The conservative-leaning Koch organization has made passing tax reform a priority and has pledged to spend tens of millions of dollars organizing voters and pressuring elected officials to support an overhaul.

In the ad, a woman looks at the camera and says that tax reform will help "average" Americans.

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"People are sick of politics. I am too. But fixing our broken tax system isn’t about politics — it’s about helping people," she says. "That’s what tax reform will do. So, what’s stopping us?"

Donnelly is one of three Democrats who did not sign a letter laying out the party's demands for a Republican-crafted tax plan, something that signaled at least some interest in working with the GOP on the issue. McCaskill and Baldwin did sign the letter but have not indicated that they would support a Republican plan.

Levi Russell, spokesman for Americans for Prosperity, a conservative advocacy group, said the ads are meant to build support for a tax overhaul.

“The American people are hungry for results, and achieving tax reform may be the last big opportunity this year," he said. "We’re urging senators to unify around the tax reform framework because it’s good policy, but it’s also good politics. There will be a lot of very unhappy folks in Missouri, Indiana, and Wisconsin if their senator is seen as protecting a rigged system and higher rates."

Wisconsin Democratic Party chair Martha Laning shot back at the ads in a statement, saying, "out-of-state billionaires are attacking Tammy Baldwin on Wisconsin airwaves because they know she won't support a tax giveaway for CEOs and the super-rich paid for by hardworking Wisconsinites."

Republicans last week released an outline of their tax plan that cuts the corporate tax rate, reduces the number of tax brackets and eliminates many individual deductions. Their goal is to release a text of the proposed legislation by Nov. 13 and pass tax reform before Christmas.

Now that the long-awaited outline has been released, special interest groups, public interest groups and the corporate world are expected to ramp up their lobbying efforts to ensure that tax reform benefits their constituency.

The ad buy is starting a week before the Koch donor network is holding a politics and policy planning session in New York City where Vice President Mike Pence is the scheduled keynote speaker.

CORRECTION (Oct. 5, 2017, 10:30 a.m.): An earlier version of this article misidentified some of the Democratic senators who signed a letter of principle on tax reform sent to Republican leaders. Joe Donnelly, who was listed as having signed the letter, did not do so. Claire McCaskill and Tammy Baldwin, who were listed as not having signed it, did sign the letter.