As Republicans have battled for the soul of their party in primaries across the country, the deep-pocketed, anti-tax Club for Growth has proved itself a force to be reckoned with.
If heavily favored Rep. Jeff Flake prevails Tuesday in Arizona over businessman Wil Cardon, the Club will mark four wins against two losses among its favored candidates in U.S. Senate GOP primary races.
Of about five-dozen organizations that spent a combined $32 million on independent expenditures in Republican Senate primaries this year, Club for Growth ranks No. 1, having spent $10.8 million, according to a Center for Public Integrity analysis of Federal Election Commission records.
The Club's super PAC, which is allowed to accept unlimited contributions and spend the funds on ads attacking or backing candidates, is responsible for nearly all of this spending.
Flake has cultivated a reputation in Washington as an anti-earmark crusader, routinely earning a 100 percent favorable rating from the Club for his voting record. He was also one of the first senators to sign the Club's pledge to repeal President Barack Obama's signature healthcare reform law.
Flake was the first candidate of the 2012 election cycle endorsed by the group back in February of 2011.
The Club has spent more than $1 million supporting Flake through its nonprofit, political action committee and its Club for Growth Action super PAC -- that's about 70 percent of the $1.5 million spent by outside groups on independent expenditures in the Arizona contest.
Cardon has sunk about $9 million of his own funds into the race, but trailed Flake by 22 percentage points in a recent poll.
Notably, the Club for Growth's PAC has also steered about $885,000 in bundled contributions into Flake's campaign war chest, according to the nonpartisan Center for Responsive Politics -- more money than any other federal candidate.
Super PACs, which have flourished in the wake of the U.S. Supreme Court's Citizens United ruling in 2010, are required to disclose their donors.
Among the super PAC's top donors are billionaire tech pioneer and Ron Paul supporter Peter Thiel ($2 million); board member and leveraged buyout specialist John W. Childs ($1.1 million); investor Virginia James, who sits on the Club's leadership council ($1 million); and board chairman Jackson Stephens ($1 million).
Club for Growth spokesman Barney Keller said his organization is "proud to play a role" in helping to elect conservatives who will "fight to pass pro-growth policies."
"We don't care about our win-loss record," Keller said. "We just want limited government, lower taxes and pro-growth policies passed in Washington."
This outlook has often put it at odds with the Republican establishment.
Club for Growth Action spent more than $5.6 million on ads aimed at helping Ted Cruz in the Texas GOP Senate primary. Cruz beat Lt. Gov. David Dewhurst in a heated primary runoff election in July. And the Club spent seven figures assisting the candidacy of Richard Mourdock, who bested moderate incumbent Sen. Dick Lugar in Indiana's Republican Senate primary.
Only in Wisconsin did an establishment candidate prevail over the Club's favored contender. Earlier this month, former four-term Gov. Tommy Thompson defeated Club-endorsed Mark Neumann, a former congressman, and political newcomer Eric Hovde, who spent more than $5 million of his own money on the race.
The Club's other loss came in Nebraska, where state Sen. Deb Fischer unexpectedly triumphed over Club-endorsed Don Stenberg and Jon Bruning, the candidate preferred by the U.S. Chamber of Commerce.
Reity O'Brien contributed to this report.