Celso Duque spent a recent summer day attaching doors to the bodies of sedans made at General Motor’s Detroit-Hamtramck Assembly plant, where he’s worked for 22 years. Until early June, he was president of United Auto Workers Local 22, but he decided not to run for reelection after GM announced plans to idle the plant in 2020. Duque moves from task to task — wherever he’s needed in the factory — as its workforce has dwindled.