Out of all the competitive Senate races this cycle, former Wisconsin Gov. Tommy Thompson (R) is emerging as a leading target of conservatives.

While candidates in other races could still emerge, conservative grassroots groups have failed to find top-tier recruits against centrist Republicans in some other Senate races, leaving them to focus their ire on the former governor and Health and Human Services secretary.

Tea Party groups had once talked about targeting Sens. Olympia Snowe (R-Maine) and Bob CorkerRobert (Bob) Phillips CorkerDeficit hawks voice worry over direction of tax plan The Hill Interview: Budget Chair Black sticks around for now Overnight Finance: White House requests B for disaster relief | Ex-Equifax chief grilled over stock sales | House panel approves B for border wall | Tax plan puts swing-state Republicans in tough spot MORE (R-Tenn.), and had hoped a strong, down-the-line conservative would emerge against centrist former Rep. Heather Wilson (R-N.M.). None of those things has happened.

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They failed to recruit Rep. Jason ChaffetzJason ChaffetzOvernight Finance: Trump pitches massive tax cuts | Freedom Caucus endorses plan | Dems slam framework | House GOP to move B border wall bill | Officials under fire for private jet use GOP lawmaker pushes to end sports leagues' tax-exempt status Republicans predict Senate ObamaCare repeal would pass House MORE (R-Utah) to run against Sen. Orrin HatchOrrin Grant HatchGOP eyes limits on investor tax break Children’s health-care bill faces new obstacles Overnight Finance: White House requests B for disaster relief | Ex-Equifax chief grilled over stock sales | House panel approves B for border wall | Tax plan puts swing-state Republicans in tough spot MORE (R-Utah) and had hoped that Indiana State Treasurer Richard Mourdock (R) would have emerged as a stronger challenger to Sen. Dick Lugar (R-Ind.) than he has so far proven himself.

Thompson, on the other hand, has a serious challenger in former Rep. Mark Neumann (R-Wis.), for the nomination to run for retiring Sen. Herb Kohl’s (D-Wis.) seat.

Neumann is running to Thompson’s right and last week secured the endorsements of Sens. Rand PaulRandal (Rand) Howard PaulHouse bill set to reignite debate on warrantless surveillance Authorizing military force is necessary, but insufficient GOP feuds with outside group over analysis of tax framework MORE (R-Ky.) and Tom CoburnTom Coburn-trillion debt puts US fiscal house on very shaky ground Al Franken: 'I make fun of the people who deserved it' The more complex the tax code, the more the wealthy benefit MORE (R-Okla.), two favorites of the Tea Party. The deep-pocketed, fiscally conservative Club for Growth has already made Thompson a target.

Neumann may be conservatives’ best hope to take out a big-name Republican with a record of breaking with party orthodoxy — something Tea Partiers did with much success in the 2010 primaries. Thompson’s biggest offense was backing early incarnations of President Obama’s health insurance overhaul law.

Still, while the Club for Growth is strongly backing Neumann, other organizations like Citizens United, Freedomworks and Sen. Jim DeMint’s (R-S.C.) Senate Conservatives fund have yet to endorse in the race, indicating some hesitation about the candidate.

Club for Growth spokesman Barney Keller said it’s not too late to get a strong conservative candidate in other races like Utah, and mentioned Texas and Arizona as other states the group was involved in in Senate primaries. But he admitted that how other races have developed has affected the group’s focus on Wisconsin.

“We look at the map and look at where we can make a difference, and we don’t make our decisions in a vacuum,” he said.

A spokesman for Thompson’s campaign blasted the Club for Growth for its involvement in the race.

“Wisconsin primary voters know Tommy Thompson as the Republican reformer that cut taxes, created jobs, reformed welfare and started school choice. Those were landmark victories for conservatives that resonate with the base and cement his credibility as the state’s next U.S. senator,” said Darrin Schmitz. “No amount of D.C.-based smears will change that.”

Chip Englander, a Neumann adviser, said he expects the campaign to solidify more conservative leaders’ support. “There’s no doubt that Mark Neumann is one of the premier conservative candidates in America in any race,” he said. “People are going to want to look at him and get him in there.”

A spokesman at another conservative group, who declined to be named because the group hasn’t yet endorsed in the race, called Thompson a “nonstarter” but expressed some concern about Neumann’s 20-point primary loss in the 2010 gubernatorial primary to now-Gov. Scott Walker (R).

“This is clearly one of the [races] where there’s going to be a strong conservative alternative,” said the spokesman, who added that the group was weighing endorsements for both Neumann and state House Speaker Jeff Fitzgerald (R), another Republican running to Thompson’s right.