The battle to decide the Republican candidate for Wisconsin's open U.S. Senate seat is heating up. The Super PAC for the national organization Club for Growth [3], which supports Mark Neumann [4] in the four-way primary, recently spent $700,000 airing an ad [5] attacking Neumann's top challengers: multi-millionaire hedge fund manager Eric Hovde and former Wisconsin Governor Tommy Thompson. Both have a significant lead [6] over Neumann (the ad ignores the fourth primary candidate, Wisconsin Assembly Speaker Jeff Fitzgerald).

The ad attacks Thompson for proposing "nine different tax hikes" as governor of Wisconsin, including raising the gas tax and a tax on nursing home beds, then shifts to attack Hovde, whom the ad says "is like Thompson, only worse." Hovde recently supplanted Neumann in the number two position [7] behind Thompson in the GOP Senate primary. The ad claims Hovde supported "tax-raising Governor Jim Doyle" and that "in a national TV interview, Hovde said he has no problem with raising taxes." (What he actually said in his 2009 CNBC appearance [8] was "I have no problem with me getting charged higher taxes. I've been blessed.")

Under the Supreme Court's Citizens United decision, Club for Growth's Super PAC, called Club for Growth Action, can raise and spend unlimited amounts of money to influence elections, but it cannot coordinate with candidates. The Super PAC has undertaken a national strategy of furthering a "starve the beast" anti-tax agenda by attacking prominent candidates from the right. In 2011, the Club worked hard to unseat long-term incumbent and moderate Republican Senator Dick Lugar [9] in the Indiana Republican primary who lost to hard-liner Richard Mourdock, the Indiana State Treasurer. It also played a central role in the election of Florida's Marco Rubio and Pennsylvania's Pat Toomey.

Hovde quickly fired back at the Club for Growth ad, telling the Daily Caller [10] he was "incredibly disappointed in the Club for Growth's decision" to attack him. "You know, I thought the Club for Growth was supposed to be about the philosophy instead of personal relationships," Hovde told The Daily Caller, "but that's all it's come down to." Hovde says that several Club for Growth staffers are tied to Neumann. R.J. Johnson [11] recently led Club for Growth-Wisconsin's chapter and also managed Mark Neumann's 1998 U.S. Senate race (current information about Club for Growth-Wisconsin's leadership is not available).

Hovde has also released an ad [12] criticizing Neumann for the Club's attacks and questioning his conservative credentials. Curiously, Hovde's ad is responding to a Club for Growth spot and uses a snippet from the ad, but attributes the attack to Neumann.

The Wisconsin Republican Senate Primary is scheduled for August 14th. The winner will face Rep. Tammy Baldwin in the general election.