Americans for Prosperity, the Koch brothers-backed conservative powerhouse, seems to have no problem sweating the small stuff.

The group is wading into Wisconsin municipal politics, working to derail a Milwaukee streetcar project that has been a source of conflict between Gov. Scott Walker (R) and Milwaukee Mayor Tom Barrett (D) for years.

Walker’s opposition to the streetcar dates back to his time as Milwaukee County executive. Barrett, who lost the 2010 gubernatorial race to Walker and was again defeated in a recall election in June, has been pushing since 2004 for a system of light rail-style streetcars that would link the lower east side of his city to the downtown Amtrak-Greyhound station. The city council approved it in 2009. Walker was Milwaukee County executive from 2002 to 2010.

AFP has collected more than 2,500 signatures for an online petition launched last week opposing the streetcar in hopes of putting the project up for referendum in November.

The project would cost $65 million in mostly federal funds, but that’s too much for AFP, the nonprofit founded by billionaire brothers Charles and David Koch.

“We don’t see the benefits,” said Luke Hilgemann, the organization’s Wisconsin state director, citing a study from the Madison-based MacIver Institute that found the streetcar would cost an additional $55 million to move utility wires. “We think it’s a waste. … They mayor is ramming it through.”

AFP is a major player in political spending this election cycle and expects by early next month to have spent at least $55 million on advertisements opposing Democratic policies this cycle, including a $25 million campaign against President Barack Obama.

Walker and Barrett squabbled over the streetcar proposal in debates and advertisements during a high stakes recall election fueled by millions of out of state dollars on both sides.

AFP ran no express advocacy advertisements during the race, but spent some $10 million on issue- focused ads and organizing grass-roots activities in support of Walker’s budget proposal in the year prior to the recall election, according to Levi Russell, a spokesman for the group.

Hilgemann said that the group was not acting on behalf of Walker and that AFP decided to oppose the streetcar in response to concerns from activists in the Milwaukee area.

Still, it is unusual for a national interest group to insert itself in a local transportation fight, industry experts said. And, that’s not lost on Barrett’s office.

“You don’t see them stopping light rail in Houston or Dallas or other communities,” said Patrick Curley, Barrett’s chief of staff. “Politically, these guys are just looking for an opportunity to kick the hell out of Milwaukee. They have a long relationship with the governor.”