"It's not surprising that a liberal Republican who supported big labor's agenda in Congress would raise money from his allies in big labor," Barney Keller, a spokesman for the Washington-based Club for Growth, said in a telephone interview. "It ain't exactly dogs and cats living together. It's more like birds of a feather."

In an odd pairing, given its funding source, Main Street is working alongside the U.S. Chamber of Commerce, the nation's largest business lobby, to defend Republican candidates deemed more practical and economic-minded over the Tea Party recruits.

The union financing for Main Street shows that some labor leaders are hedging their partisan bets, recognizing that Republicans are likely to hold a majority in the House after the midterm elections and bipartisan allies could be beneficial later.

Jay Lederer, communications director for the International Union of Operating Engineers, said that transportation and infrastructure projects that provide jobs are getting "bogged down in this extreme polarization we've seen" in Congress.

"We see Defending Main Street as one group and one way of part of our overall political program to try and get some more folks elected to Congress who are going to work together to get things like that done, instead of putting these things off in continuing resolutions year after year after year," Lederer, whose union has donated to the super-PAC, said in an interview.

Sarah Chamberlain, the chief operating officer of the Republican Main Street Partnership, said it's a "win-win" situation for her organization because the labor money will help defend Republican seats. "Why not take union money to maintain a majority in the United States House?" she said.

Labor Split

In some races, the result may be that labor is on both sides of a contest, with some union money going to the Democratic nominee in a Republican-held district that the super- PAC also is targeting.

Main Street plans to focus most of its resources on defending competitive House districts held by retiring Republicans, including one district in suburban Philadelphia, one in south-central New Jersey and another in northern Virginia near Washington, D.C., Chamberlain said. Democrats are also targeting those three districts as they seek a 17-seat net gain needed to overturn the Republican House majority.

Jesse Ferguson, a spokesman for the Democratic Congressional Campaign Committee, didn't respond to an e-mail seeking comment on the prospect of clashes between candidates his group backs and Republicans supported by union-funded Main Street.

Labor's political activity has long tilted Democratic, though not exclusively so. In the 2012 campaign, unions gave $61.1 million to Democratic candidates and committees, compared with $6.1 million to Republicans, a ratio of 91 percent to 9 percent, according to the Center for Responsive Politics, a Washington-based group that tracks political giving.

Thus far in the 2014 election, labor groups have given $17.5 million to Democrats and $2.4 million to Republicans, a ratio of 88 percent to 12 percent.

The Main Street super-PAC and an allied nonprofit organization have raised about $2 million toward an $8 million goal, Chamberlain said. Super-PACs may raise money in unlimited amounts to fund television and radio commercials that directly advocate for the election or defeat of federal candidates.

It received $250,000 each from Working for Working Americans, a super-PAC associated with the United Brotherhood of Carpenters and Joiners; the International Union of Operating Engineers; and the Laborers' International Union of North America.

Idaho Primary

While the Main Street super-PAC wants to focus on defending districts of retiring Republicans, its current top priority is aiding one of its House allies.

The super-PAC is prepared to spend up to $1 million to help Idaho Representative Mike Simpson fend off a Republican primary challenge from Bryan Smith, a lawyer backed by the Club for Growth and other Tea Party-allied groups. The Chamber of Commerce is also supporting Simpson.

The super-PAC also received $50,000 from the Chickasaw Nation, an American Indian tribe based in Ada, Oklahoma, and $30,000 from David Bonderman, a founding partner of TPG Capital, a Fort Worth, Texas-based private-equity firm.

Bonderman has donated hundreds of thousands of dollars to Democratic candidates and committees within the past four years, including $175,000 to a super-PAC that helped Democrats defend their Senate majority in the 2012 election.

Chickasaw Nation didn't return requests for comment. Bonderman declined to comment, Owen Blicksilver, a spokesman for TPG at Owen Blicksilver Public Relations Inc., said in an e- mail.