The Republican Governors Association, led by Virginia Gov. Bob McDonnell, blasted Maryland Gov. Martin O'Malley on Thursday as a tax-happy liberal in an ad running in the Wisconsin governor's race that is fast becoming a proxy war between McDonnell and O'Malley.

The Republican attack on O'Malley, leader of the Democratic Governors Association, in the one-minute Web video was the latest indication that the once-friendly rivalry between the two Beltway governors is turning nasty as they prepare to square off this fall on opposite sides of nearly a dozen competitive gubernatorial races.

The video shows a series of news clips about O'Malley's push to raise taxes in Maryland and links his record to Milwaukee Mayor Tom Barrett, the Democratic nominee hoping to oust Republican Gov. Scott Walker from office in Wisconsin's recall election next week.

"The largest tax hike in state history. A 'tax-raising legacy.' Taxpayers moving to other states," the ad says. Then, a picture of Barrett pops up. "Sound familiar?"

The ad was released just as O'Malley was preparing to fly to Wisconsin to campaign with Barrett. McDonnell was in Wisconsin earlier this week, appearing with Walker.

The governors associations McDonnell and O'Malley lead are expected to be among the highest-spending groups in the Wisconsin election, the first contest in which the two local governors have gone head to head.

Barrett's campaign and the Democratic Governors Association did not return messages seeking comment.

McDonnell and O'Malley insist publicly that they're friends, often discussing their shared Irish heritage and willingness to work together on regional issues. But with every joint appearance on Sunday morning political talk shows, their tone has grown increasingly aggressive and their relationship appears to be deteriorating.

"Both these governors have national reputations at this point. They're respected by their peers, they're in demand on the campaign trail, and both are interested in building up their standing in their parties," said Stephen Farnsworth, political science professor at the University of Mary Washington. "In a way, it's surprising the truce between these two governors lasted as long as it did.

"It's far more likely to get worse than better between here and November."

Wisconsin's recall race will set the tone for 11 more gubernatorial elections this fall.

McDonnell's group has already spent $8.3 million to help Walker ahead of Tuesday's election. The Democratic group led by O'Malley spent $3.2 million to help Barrett in what is essentially a rematch of Wisconsin's 2010 gubernatorial race.