The billionaire brothers behind the Tea Party and conservative political organizations and think tanks have put their money where their mouths are in the 2012 presidential campaign.

Late last month, Americans for Prosperity–a conservative organization backed by oil and gas moguls Charles and David Koch of Koch Industries–unveiled a $6.1 million advertising onslaught aimed at key swing states in the election. The ads accuse President Barack Obama of wasting billions of dollars working on green energy. They particularly target the federal loan guaranteed to alternative energy company Solyndra, which was recently forced into bankruptcy. The ads aired in Colorado, Florida, Iowa, Michigan, Nevada, New Mexico, Ohio and Virginia.

It’s the second wave of attacks on Obama by Americans for Prosperity, and David Axelrod, Obama’s campaign advisor, didn’t take too kindly to the Koch brothers making their presence felt in the presidential election process.

During a short telephone news conference, Axelrod told reporters that Mitt Romney is being aided by “the Karl and Koch brothers’ contract killers in super PAC land.”

“We’re going to be tough about that, even as we make our positive case to the American people,” said Axelrod. The ‘Karl’ part of the ‘Karl and Koch brothers’ statement is a reference to conservative political strategist Karl Rove.

The Koch brothers fired back at Axelrod and the Obama administration in a statement.

“Despite pronouncements from the Obama campaign that its rhetoric would become more positive, senior Obama advisor David Axelrod’s comments to reporters on May 7 reveal that the campaign continues to stoop to the lowest forms of invective,” said Philip Ellender, the president of governmental and public affairs for Koch Companies Public Sector, LLC. “Mr. Axelrod described Koch as ‘contract killers over there in super PAC land.’”

“Koch has not contributed to any super PAC organizations as of yet in this election cycle,” Ellender continued. “Our criticisms have been directed at policies of this administration that have looked to big government solutions and have led our country into unsustainable debt. We believe that economic freedom and free markets are the best means to advance societal progress.”

Obama’s camp responded in kind.

“Secretive oil billionaires are making good on their promise to spend hundreds of millions of dollars on Governor Romney’s behalf, attempting to defeat the president,” said campaign spokesman Ben LaBolt in a statement.