The political arm of the network helmed by conservative and libertarian mega donors Charles and David Koch is investing millions in an ad buy targeting two Democratic Senators up for reelection this year in states President Trump won by a landslide for failing to support the tax reform bill.

The group, Americans for Prosperity, is launching the $4 million ad campaign on Feb. 15 against Missouri Senator Claire McCaskill and Indiana Senator Joe Donnelly.

“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 deserve better, which is why AFP is committed to ensuring citizens see the pro-growth benefits of tax reform despite dismissals and deception from ‘no’ votes like Donnelly and McCaskill,” AFP President Tim Phillips said in a statement.

Both ads tout the economic benefits of tax reform in Indiana and Missouri, and pointedly note that both Senators stood with Democratic leadership; the ads specifically call out Senate and House Minority Leaders Chuck Schumer and Nancy Pelosi.

“Senator Claire McCaskill said she’d support tax cuts for hard working Missourians,” the ad against McCaskill states, “but when she had the chance, she said no, voting against tax cuts for you.”

The Koch network had already invested $20 million in pushing for the passage of the tax reform bill, and had announced at its annual winter gathering in California last month that it was planning on spending an additional $20 million to sell it. In total, the politics and policy arm of the network could spend a record setting $400 million for the upcoming midterm elections.

But the massive investment comes in a political environment that is highly unfavorable to Republicans. Even though Trump won Missouri and Indiana by almost 20 points, Democrats are expected to emerge as the victors in what could be a wave election, regaining control of the House of Representatives. The map is less favorable to the Democrats in the Senate, and the non-partisan Cook Political Report has rated McCaskill and Donnelly’s seats as toss-ups. But both candidates had significantly out-raised their potential Republican counterparts by the end of 2017, according to disclosure records from the Federal Election Commission.

These are all factors that officials with the network have openly acknowledged.

“These elections are going to be brutally tough,” Emily Seidel, Chief Executive Officer of Americans for Prosperity, said at the seminar last month. “We’ve never faced a challenge like this one.”

— With reporting by Phil Elliott

(Disclosure: Time Inc., TIME’s parent company, has been acquired by Meredith Corp. in a deal partially financed by Koch Equity Development, a subsidiary of Koch Industries Inc.)

Get our Politics Newsletter. The headlines out of Washington never seem to slow. Subscribe to The D.C. Brief to make sense of what matters most. Please enter a valid email address. Sign Up Now Check the box if you do not wish to receive promotional offers via email from TIME. You can unsubscribe at any time. By signing up you are agreeing to our Terms of Use and Privacy Policy . This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply. Thank you! For your security, we've sent a confirmation email to the address you entered. Click the link to confirm your subscription and begin receiving our newsletters. If you don't get the confirmation within 10 minutes, please check your spam folder.

Write to Alana Abramson at Alana.Abramson@time.com.