Detroit ? The U.S. auto industry is gasping for air under the crushing weight of high labor costs, growing competition and bloated, aging infrastructure. At the same time, the United Auto Workers union is struggling to stay relevant after years of membership declines.

Caught between these two giants in this year’s contract talks are the auto workers who built middle-class lives on the promises of the industry. Marilyn Coulter, who has built cars in Lansing for 29 years, said nobody likes to strike. But she was still holding a picket sign outside an entrance to her sprawling plant Monday.

“We do what we have to do. We’ve given and we need something back,” said Coulter, 47. “We want to build cars, but we don’t want to be taken advantage of, either.”

Strikes have sometimes been jubilant affairs, with workers lustily singing and chanting on the picket lines. But on Monday, UAW President Ron Gettelfinger and his bargaining team looked exhausted and grim. Gettelfinger said the union called a strike because it wants promises for future products at U.S. plants in addition to standard agreements on wages and benefits.

“Nobody wins in a strike, but there comes a point in time when someone can push you off a cliff,” said Gettelfinger, who represents 73,000 GM workers at about 80 U.S. facilities.

GM acknowledged that the job security of U.S. workers is on the line, but so is the long-term viability of the company. The two sides went back to the bargaining table Monday afternoon, but weary negotiators stopped to rest about 7 p.m. CDT. Talks were to resume this morning.

Several analysts said they expect the strike will be short and that the UAW is simply trying to force GM’s hand and bring the lengthy bargaining to a close. GM and the UAW had been negotiating for 21 straight days as of Monday.

Gary Chaison, a labor specialist at Clark University in Worcester, Mass., said it was a good sign that the two sides are still talking. But he added that the strike isn’t all drama.

“There’s always theatrics in bargaining, but the theatrics are very real,” he said. “These are workers whose jobs are in jeopardy and the company could lose millions and millions of dollars.”

The striking workers will receive $200 a week plus medical benefits from the UAW’s strike fund. The union had more than $800 million in that fund as of last November, according to the UAW’s Web site.

Aaron Bragman, an auto industry analyst for the consulting firm Global Insight, said the two sides are at a historic crossroads. GM, which lost $2 billion in 2006, isn’t in the financial position to make long-term promises for U.S. manufacturing, and the company could decide to move products overseas if the strike lasts longer than a few days, he said.

Detroit’s automakers lost a collective $15 billion last year. When labor talks began in July, all said they were seeking labor cost parity with their Asian competitors in order to turn consistent profits. GM said it pays its workers $73.26 per hour in wages, pension and health care benefits, while Toyota Motor Corp. pays an estimated $48 per hour, according to an estimate from U.S. automakers. Retiree health costs are a large part of that gap.

GM also is facing a changing landscape in which barely half the vehicles on U.S. roads are made by the traditional Big Three. GM controlled 35 percent of the U.S. market in 1990; that fell to 24 percent last year.