By SHARON COHENFrom a sprawling United Auto Workers hall outside Detroit, John Zimmick has seen factories close and grown men cry when their jobs disappear. Through all the economic uncertainties of life in auto country, there has been one constant: the union. In its nearly 80-year history, Zimmick's UAW Local 174 has been tested by bitter strikes, foreign competition and tenacious opponents. Now comes a new reason for anxiety. On Thursday, Michigan's right-to-work law takes effect, a stunning shift in this symbolic capital of organized labor. The historic change is just the latest sign of turmoil in the union movement that has seen its nationwide membership shrink to its lowest levels since at least the 1930s â¿¿ a paltry 6.6 percent in the private sector. With 14.4 million members, unions still can be a potent political force at the ballot box. But protests in recent years over the passage of right-to-work laws in Michigan and Indiana, clashes over collective bargaining in Wisconsin and Ohio and a sharp drop in union elections across the US have raised larger questions: Where do unions go from here? How they do mend their battered image? Can they recruit new members? And is organized labor even a movement any longer? Zimmick looks for answers in a union hall steeped in history. It's filled with photos, meeting minutes and other memorabilia belonging to Local 174's first president, Walter Reuther â¿¿ even a phone used by the legendary leader who transformed the UAW into an economic and political powerhouse. Modern-day realities are far different: With layoffs and some 30 plants closing in the last five years, the local's ranks have dropped by more than a third, to about 5,000. There could be even more losses with right-to-work, signed into law last December by Michigan Gov. Rick Snyder. Though employees won't have to make mandatory payments to unions that represent them in collective bargaining agreements, Zimmick isn't expecting the measure to have a major impact. "It's going to weaken us," he says, "but it's not going to kill us."