Those issues are central to the AFL-CIO Building and Construction Trades Dept.'s criteria for support, says spokesman Tom Owens. Compared with some individual unions, the BCTD's spending in congressional races is more evenly divided between the parties. Owens says there are about two dozen House Republicans who back PLAs and nearly 50 who support preserving Davis-Bacon protections. "We look at each individual member, and if they're supporting us on our core issues, then we're going to support them as well," he says.

Most of the unions have endorsed Obama in the presidential contest. Jay Lederer, spokesman for the International Union of Operating Engineers, says, "We believe an Obama administration will have a stronger commitment to make the investments our country needs in roads, bridges, rail, ports and other transportation and infrastructure projects."

Unions acknowledge that they can't match businesses' total financial contributions. But they say they have another key strength—their thousands of members who can staff phone banks and other get-out-the-vote campaigns for their candidates. Greer says, "Our greatest resource is our membership and their family and friends, and that's where we're really putting our focus and our emphasis."

Owens says that initially, it was difficult to get BCTD members engaged and excited about Obama's re-election campaign because of construction's continuing high unemployment levels, particularly for unionized workers. But AFL-CIO polling indicates rank-and-file union members are getting more active.

"It's kind of like in 2008," Owens explains. "Senator Obama was kind of an unknown quantity, and our guys were slow to come around, but in the end, they really came around strong, particularly in the battleground states, and we're starting to see that in this cycle as well."