When a prankster called pretending to be billionaire David Koch, Gov. Scott Walker took 20 minutes out of his packed schedule to provide an unguarded briefing on the political stand-off in Madison.

No wonder: Koch is one of the biggest financial backers of conservative causes in the country, including Walker's campaign for governor.

David Koch (pronounced "coke"), the man impersonated in the phone call to Walker, and his brother Charles control Koch Industries Inc., a privately held company with estimated revenue of $100 billion a year.

Last year, Koch Industries' political action committee contributed $43,000 to Walker, the largest donation the Republican candidate received from a corporate committee, and the group's largest gift in Wisconsin.

In addition, David Koch, executive vice president of the company, wrote a check for $1 million to the Republican Governors Association last year. Koch Industries also contributed an additional $50,000 to the governors' group.

The Republican Governors Association spent $65,000 on ads supporting Walker and an additional $3.4 million on TV and radio spots attacking Mayor Tom Barrett, Walker's Democratic opponent.

"They're clearly right-wing kingmakers on the national stage," said Michael McCabe, executive director of the Wisconsin Democracy Campaign, which tracks political contributions in the state. "It's also clear that they decided to invest in Scott Walker."

Koch Industries, based in Wichita, Kan., has a significant presence in Wisconsin, with operations that include pipelines, coal shipping and paper mills. Most of its 3,000 employees are employed at its two paper mills in Green Bay acquired five years ago when the company bought Georgia-Pacific Corp.

Nationwide, Koch employs about 70,000 employees in such businesses as pipelines, refineries, polymers and fibers, process and pollution control equipment, fertilizers and ranching.

It also is a stunning success story: In 1966, the year before Charles Koch became chief executive, the company had revenue of $177 million, about $1.2 billion in today's dollars.

Politics attract attention

These days the two brothers' politics draw more attention than their business success. But David and Charles Koch have been involved in politics for more than 40 years, backing free markets, limited regulation, small government and low taxes.

David Koch, who lives in New York City, has a more public profile than his brother Charles, both in politics and philanthropy, pledging $100 million to the Lincoln Center in 2008 and reportedly giving a total of more $40 million to Memorial Sloan-Kettering Cancer Center.

Initially, they focused on supporting libertarian and conservative think tanks, including helping to found the Cato Institute. But in the past 15 years, they became increasingly involved in partisan politics, including contributing to Republican candidates and supporting Americans for Prosperity, a citizen advocacy organization that backs Walker.

Americans for Prosperity Foundation is chaired by David Koch and was co-founded by Richard Fink, an economist who oversees legal, government and public affairs for Koch Industries.

Tim Phillips, executive director of Americans for Prosperity, spoke at Saturday's tea party event at the Capitol in support of Walker. It also launched StandwithWalker.com, a website encouraging supporters of the governor's budget-repair bill to sign petitions.

The group's state chapter also played a key role in organizing last year's tea party rallies.

"Koch Industries has a long history of support for political action groups and think tanks hostile to public employee unions," said Jay Heck, executive director of Common Cause in Wisconsin.

Koch Industries' PAC, which contributed to both sides of the aisle for much of the past decade, gave $10,000 to Republican Attorney General J.B. Van Hollen in 2007 and $6,000 to conservative Supreme Court Justice Michael Gableman in 2008.

Even so, the company's political fund gave $3,000 each in 2009 to the Assembly Democratic Campaign Committee and the State Senate Democratic Committee, the fund-raising arm for the 14 Senate Democrats who fled the state to prevent their Republican colleagues from pushing through Walker's budget-repair bill.

Koch Industries has established an active lobbying operation in Madison, headed up by Jeff Schoepke, a former lobbyist with Wisconsin Manufacturers & Commerce, the state's largest business lobby.

State records show that a Koch Industries affiliate spent $396,233 on lobbying in the state during the 2009 and 2010 legislative session - the last period for which financial figures are available. The company now has seven lobbyists in Madison.

Critics of Walker's budget-repair bill have focused on its plan to strip most collective bargaining rights for public-sector employees. But some raised concerns this week about a no-bid clause that would enable the state secretary of administration to sell off more than 30 plants that heat and cool prisons, university campuses and other government buildings.

Walker's critics have suggested the governor planned to reward Koch Industries by selling the heating plants to the company.

Koch Industries said it has no interest in buying the plants.

"Any allegations to the contrary are completely false," Philip Ellender of Koch Industries said in a statement.

Jeff Plale, a former Democratic state senator who was hired by the Walker administration to run the Division of State Facilities, said no one has shown interest in buying the plants, including Koch Industries.

"I wouldn't know them if they walked in here and sat at my desk," Plale said of Koch.

Daniel Bice of the Journal Sentinel staff contributed to this report.