Six Democratic presidential contenders, courting one of the party's most crucial interest groups, pledged Wednesday to work to reverse decades of decline in the nation's union movement.

Sens. Joe Biden, Hillary Rodham Clinton, Chris Dodd and Barack Obama, former Sen. John Edwards and New Mexico Gov. Bill Richardson said they would support labor-backed issues, such as expanding health care and toughening trade rules.

However, they contended that the biggest problem facing unions was the steady drop in membership, and all vowed to make it easier for unions to organize.

"It is not rich people that made this country great," Clinton said. "We would not have had a middle class without the American labor movement."

Edwards sounded a similar theme: "If we want to grow and strengthen the middle class, we have to grow the union movement."

Labor leaders grilled each of the candidates at the annual convention of the Iowa Federation of Labor, a key player in the state's Democratic politics. Each candidate had a half-hour session, and they were often dragged through the arcane details of policies backed by labor.

The umbrella labor group is unlikely to formally endorse a candidate for Iowa's leadoff precinct caucuses, but the more than 250 activists who listened to the candidates are all local political leaders. The federation represents roughly 55,000 workers, and it's likely that about 100,000 Democrats will settle next winter's caucuses.

"As long as I'm alive and breathing, I will be standing with you," said Edwards, who joined Obama in pledging to walk a picket line if elected.

"Think about a president of the United States walking a picket line," Edwards said. "You're looking at one."

All the candidates noted their long ties to unions.

"The AFL-CIO in New York has dubbed me their favorite sister," Clinton said. "I will use the bully pulpit as president."

If elected, Obama promised to include organized labor when he shapes his new administration.

"You have to have a president who is willing to talk about unions," he said. "I'd make sure the AFL-CIO was involved in the transition process."

Biden called for easing restrictions on union organizing and noted the decline in membership from the 1950s, when roughly one in four workers belonged to a union.

"You are the middle class," he said. "If you want to grow the middle class, you have to grow the union movement."

All agreed that a series of trade deals has given competitors an unfair advantage and shifted jobs overseas.

"There ain't no such thing as free trade unless its fair trade, and that's not what's happening now," Biden said.

Dodd called for new restrictions on toys and food imported into the U.S. from China, pointing to reports of contaminated food and recalls of dangerous toys.

"This is an issue of safety," he said. "Parents should be confident that the toys they give their children have been inspected and are safe."

Richardson said he would set concrete goals for increasing union membership from its current 12 percent of the work force, pushing for a level as high as 20 percent.

"My secretary of Labor would be a union member," he said. "All of those union-busting attorneys would be gone."