Wichita (ap) — A bitterly contested Republican congressional race unfolding in south-central Kansas is testing the political influence of big corporate money in the backyard of two billionaire brothers who have poured millions into races across the nation to advance their agenda of low taxes and less regulation.

The 4th District GOP primary Aug. 5 pits U.S. Rep. Mike Pompeo of Wichita against an unusual late-filing challenger — the popular conservative Republican who previously held that position. Former congressman Todd Tiahrt, also from Wichita, gave up the House seat in 2010 for an unsuccessful GOP primary run for the U.S. Senate against Jerry Moran.

Tiahrt, 63, wants his old House seat back, casting himself — despite a career that includes 16 years in the U.S. House — as an outsider fed up with congressional dysfunction. Pompeo, 50, says he’s on his own mission to shrink government by cutting spending and rolling back regulations.

The primary challenge comes in the home district of Charles and David Koch, who have sent much of their personal fortunes to a network of conservative organizations nationwide. Koch Industries’ employee political action committee, KochPAC, supports Pompeo and has given him the maximum allowable amount.

“They are true believers in liberty and freedom and small government,” Pompeo said. “So I am pretty convinced that I will do pretty well among those folks who share that same set of values and work at Koch Industries or any other great company here in south-central Kansas.”

Tiahrt and Pompeo accused each other during a recent forum of not being truthful, continuing personal attacks waged in press releases, ads and social media. Little separates the two conservative Republicans ideologically, though Tiahrt has criticized Pompeo over whether he’s done enough to defund the health care overhaul. He’s critical of Pompeo’s support of National Security Agency surveillance operations and Pompeo’s opposition to wind energy tax credits.

Pompeo has assured constituents the NSA isn’t listening to their phone calls or reading their emails and points to his numerous votes to repeal the health care law.

But with the Wichita economy ailing from its depressed aviation industry, Tiahrt’s sharpest barbs focus on whether Pompeo does enough for his district. Tiahrt points to federal money he brought home to public works projects and local aviation companies during his previous tenure, while Pompeo criticizes those earmarks and suggests local businesses are better served by lower taxes.

Finance reports showed Pompeo had more than $2.1 million in campaign cash at the end of March. Tiahrt didn’t announce his candidacy until May 28 and is essentially starting from scratch. Updated finance reports are due July 15 and will reveal whether Tiahrt has gained ground.

Writing off Tiahrt because of his money disadvantage is risky. Few know that like Dan Glickman, the Democrat who represented the 4th District for 18 years before Tiahrt beat him in 1994. Glickman — who later served as U.S. agriculture secretary and is now a senior fellow at the Bipartisan Policy Center in Washington, D.C. — had far more campaign money and easily outspent Tiahrt.

Tiahrt “has a very strong grassroots support, particularly from the right-to-life community and other communities as well,” Glickman said. “And while there is not a huge difference between him and Mike on any of these issues, my guess is that he will organize, recognizing that he cannot raise the money probably that Pompeo can raise as an incumbent with access to large sums of money.”

Tiahrt is getting support from Kansans for Responsible Government, a “super PAC” that’s free from campaign contribution limits because it independently advocates for his election. The PAC, formed by Kansas oilman Willis “Wink” Hartman, can’t give money directly to Tiahrt but has spent $15,500 on ads attacking Pompeo. Pompeo beat Hartman in the 2010 GOP primary.

Tiahrt said he had no Koch backing in 1994 when he beat Glickman. But he received Koch money in subsequent campaigns. During his political career, Tiahrt received $328,766 from Koch Industries-related individuals and political action committees, according to the Center for Responsive Government.

“I was accused of being the Koch congressman for awhile because Koch did support me and I was for a free market, and I haven’t changed,” Tiahrt said. “But Koch is a business and they want the best legislation that money can buy and I don’t do that kind of thing. I am for the people, not special interests.”

Koch money now goes to Pompeo. Koch Industries-related individuals and PACs have donated $60,100 between 2013 and 2014, according to the CRG. Overall, Pompeo has received $239,900 in Koch-related campaign money throughout his political career, the figures show.