Tax reform didn't help the GOP win in Pennsylvania's special House election, but Republicans are still banking on the legislation to help them maintain their majorities on Capitol Hill in November's midterm elections.

Americans for Prosperity, which is backed by the billionaire Republican Koch brothers, announced Wednesday it will sink $4 million more into new digital and TV ads touting the benefits of tax reform in the battleground states of Indiana and Missouri. Both states' Democratic senators — Joe Donnelly and Claire McCaskill, respectively — voted against the tax plan and are facing challenging re-election campaigns.

In Missouri, the ads are launching the day President Donald Trump is scheduled to visit a Boeing plant in St. Louis to talk up Republicans' signature legislative achievement.

"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," Americans for Prosperity President Tim Phillips said in a statement.

The tax push comes after a tight special election in Pennsylvania's deep-red 18th congressional District, where Trump beat Hillary Clinton by 20 points in 2016.

Conservative groups spent nearly $11 million on the battle for the seat — more than double the amount spent for Democratic candidate and projected apparent winner Conor Lamb, according to the Center for Responsive Politics. Initially, conservative-fueled ads supporting Republican state legislator Rick Saccone focused squarely on the overhaul of the tax code and the bump in worker paychecks.

But as polls showed during the campaign that Saccone was lagging behind Trump's margin, Republicans abandoned that messaging in favor of social issues, such as immigration, and the local politics of Lamb's record as a federal prosecutor.

Tax reform "shored up his base somewhat and it shored up the business base, the Chamber of Commerce voters," said Doug Kaplan, president of polling firm Gravis Marketing. "We're seeing evidence that it's not enough."

Republicans are hoping the tight race in Pennsylvania isn't a sign of things to come. The special election was called after the surprise resignation of anti-abortion Rep. Tim Murphy amid allegations that he asked his mistress to have an abortion. Nationally, Republicans have been buoyed by the growing support for the tax package.

"Tax cuts are becoming more popular with every bonus announcement," said John Ashbrook, a Republican strategist and Capitol Hill veteran who now works at Cavalry LLC. "Democrats are making a mistake if they over-read the outcome of this race as some sort of justification to take it all away."

In Missouri, McCaskill holds a narrow lead of 42 to 40 percent over Republican State Attorney General Josh Hawley, according to a recent Gravis poll. It also shows 30 percent of independent voters remain undecided on the tax plan, giving Republicans room to move the needle by convincing them of the legislation's benefits.

The new ad campaign targeting McCaskill and Donnelly represents the second installment of funding from the advocacy group. Americans for Prosperity already spent $4 million earlier this year targeting the two senators.

In response to that attack, McCaskill's campaign highlighted her attempts to broker a compromise with Republicans and the final party-line vote on the legislation.

"Claire's record of working across the aisle to get results for Missourians speaks for itself," McCaskill for Missouri Communications Director Meira Bernstein said in a statement at the time. "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."

Donnelly's campaign also defended his efforts to work with Republicans on tax reform.

"Joe tried hard to work with Republicans to craft a tax plan that helps middle class Hoosiers and doesn't cause skyrocketing deficits," said Joe for Indiana communications director Will Baskin-Gerwitz, "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."

Correction: An earlier version misspelled the name of Ashbrook's firm, Cavalry LLC.