Sign at the AFL-CIO building in Washington, D.C.
Sign at the AFL-CIO building in Washington, D.C. - 
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Renita Jablonski: The AFL-CIO is the biggest union federation in the country, and it's expected today to throw its weight behind Senator Barack Obama today. Union support doesn't mean what it used to, but Democrats hope it'll be enough to influence middle-class voters in key states. Jill Barshay has more.

Jill Barshay: Just 12 percent of the U.S. workforce is unionized. In the private sector, not even 1 in 13 workers is a union member.

So what's the endorsement of the AFL-CIO worth? Money for one thing. An endorsement can bring in millions of campaign dollars from electrical workers and engineers, for example.

Michael Lux is president of Progressive Strategies, a political consulting firm. He says union support can tip the scale.

Michael Lux: Elections are decided by very, very small margins. So any organization that can deliver 1 percent of the vote, or can influence 1 [percent] or 2 percent of the vote, is a very big deal.

The AFL-CIO says it plans to focus on influencing its own members, their families and union retirees in five battleground states: Ohio, Pennsylvania, Michigan, Minnesota and Wisconsin.

I'm Jill Barshay for Marketplace.