With his recall election looming in early June, Wisconsin Gov. Scott Walker is leaving no stone unturned in amassing his money advantage over the four Democratic candidates vying to take the governor’s mansion from him in retribution for his union-bashing law that all but stripped state employees of their collective bargaining rights. But a lot of those stones, it seems, are piled up in a single yard, that belonging to the billionaire brothers Charles and David Koch.

Among Walker’s biggest backing groups is the Republican Governors Association, which has created an entity dedicated to Walker’s defense. The Milwaukee Journal Sentinel‘s enterprising Daniel Bice reports:

The Republican governors’ group has made Walker’s recall contest one of its top priorities.

Under the name Right Direction Wisconsin, the RGA has bought slightly more than $3 million worth of TV time to air its commercials in Milwaukee, Madison, Green Bay, La Crosse and Wausau, according to political insiders.

This week, Bice writes, tax documents filed by David Koch revealed that he donated $1 million to the RGA this year, “which is running ads statewide supporting Walker and criticizing the top two Democratic candidates – Milwaukee Mayor Tom Barrett and former Dane County Executive Kathleen Falk,” according to Bice. He continues:

Koch’s $1 million contribution comes on top of the cash that he and his brother, Charles, have given to Americans for Prosperity, a conservative advocacy group founded by the pair that aired a series of ads earlier this year supporting Walker’s budget proposals.

The political action committee for Koch Industries – an energy and consumer products conglomerate based in Wichita, Kan. – also contributed $43,000 to Walker’s campaign in 2010.

Last year, a blogger embarrassed Walker by giving him a prank call in which the blogger pretended to be David Koch. Koch, a frequent target of liberals, told The New York Times at the time that he didn’t even know Walker’s name.