In fact, over the years the U.A.W. has taken two different postures toward Detroit: by turns, hard-charging adversary and strategic partner. Sometimes the union has been an unyielding bargainer, leading to strikes that have lasted for months as well as to much-derided perks, like overtime pay for union members who work less than 40 hours a week. But other times the union has worked with management to assure labor peace, raise productivity and, over the last few years, push down labor costs.

One example: when G.M. was hurting in 2005, the union agreed to establish the retiree health plan, known as a voluntary employees beneficiary association, to save the automaker $1 billion a year in health costs.

Union officials acknowledge their discomfort with the union being a major shareholder. “The reason we’ve received this equity stake is we’re trying to help the corporation survive and fund the VEBA,” Mr. Fredline said.

The Obama administration has pushed hard to bring out the U.A.W.’s cooperative side, evidenced by the union’s agreeing to a wage freeze and the no-strike clause.

But the union is likely to continue fighting future layoffs, plant closings and line speedups  there are many ways to pressure management without going on strike.

“They’ll want to maximize the number of jobs for their members, and they’ll use their influence for that,” said Sean McAlinden, chief economist with the Center for Automotive Research. “And there are other fears the industry has  they might want to force G.M. to only use unionized suppliers. And they could use their political influence to make it easier to organize the international automakers in the South.”