By Sasha Johnson
CNN Washington Bureau
CHICAGO, Illinois (CNN) -- There's a good reason why Democratic candidates court the support of organized labor.
"You often hear people talk about labor in decline, that unions are irrelevant, unions are dinosaurs, that they no longer matter, but in the political arena there's a different story," said Peter Francia, author of "The Future of Organized Labor in American Politics."
"I think organized labor is sometimes underestimated because their share of the work force, the percentage of workers who belong to unions, has dropped off precipitously," Francia said.
Chuck Crull, left, speaks with former Sen. John Edwards at IBEW Local 405 in Cedar Rapids, Iowa, on Monday.
Union households make up roughly a quarter of the electorate in most elections. Almost 60 percent of them voted for former Vice President Al Gore in 2000 and for Sen. John Kerry, D-Massachusetts, in 2004.
During the 2006 congressional elections, the AFL-CIO's political program boasted its volunteers knocked on more than 8 million doors and reached out to 30 million voters, many in the union-rich and politically crucial states of Ohio, Pennsylvania and Michigan.
Seven of the 2008 Democratic presidential contenders will gather Tuesday night at Soldier Field in Chicago, Illinois, to try to woo that support.
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