The Pontiac Assembly Center in Pontiac, Mich., is a massive, low-slung structure of concrete and corrugated green steel that squats conspicuously among the many strip malls that line one of the city’s main thoroughfares, South Opdyke Road. Locals refer to the three-million-square-foot factory, which makes Chevrolet Silverado and GMC Sierra pickup trucks, as Plant 6, because when it opened in 1972, it was the sixth General Motors manufacturing facility in this city, 25 miles north of downtown Detroit. At the time, General Motors was the world’s largest automaker. It dominated the American market, manufacturing half of the vehicles sold in the U.S. As recently as 2003, Plant 6 was running three consecutive eight-hour shifts, employing 3,000 people and making 1,300 trucks a day.

Today, Pontiac Assembly is the city’s last working auto-assembly plant, and like many of America’s car factories, it is operating at a greatly diminished capacity. By last summer, the plant was running just one shift — from 6 in the morning to 2:30 in the afternoon — having shed nearly two-thirds of its workers through a combination of layoffs, buyouts and early retirements. A few months ago, Plant 6 slowed down its assembly line and laid off another 600 employees, bringing the total number of remaining workers to fewer than 600. The factory now produces only about 230 vehicles a day.

On a clear, mild Thursday afternoon in April, I stood among the smattering of cars, mostly American-made pickup trucks and sport-utility vehicles, clustered together in a small section of Pontiac Assembly’s vast parking lot as the plant’s single shift ended and its employees trickled out. Among them was Marvin Powell, a tall, heavyset, African-American man in blue jeans, a green sweatshirt and a baseball cap that read “All-Star Dad.” We were going to throw horseshoes with some of his co-workers in a park next to their union hall, Local 594, but as Powell climbed into his Chevy Equinox, he...