Production in Lordstown fell by more than half last year from 2016. This year it has operated just a single, eight-hour shift each day. Typically, car plants must operate two shifts to generate profits.

Moreover, the outlook for each plant is grim. With gas prices low, American consumers have flocked to S.U.V.s and all but abandoned small cars, like the Cruze, which is made in Lordstown, and the Impala, the Buick LaCrosse, and the Cadillac CT6, made at Detroit-Hamtramck. The plant in Oshawa also produces the Impala.

Are these plants closed for good?

Not necessarily. G.M. specifically announced that the plants were now “unassigned” — that is, they have not yet been assigned new vehicles to make after they stop production in 2019.

Next year the company and the United Auto Workers union will negotiate a new labor contract. In past negotiations, the union has given concessions on wages and other cost-saving measures in exchange for the company’s keeping plants open. Idling the plants now gives G.M. a powerful bargaining chip in the contract talks.

“Many of the U.S. workers impacted by these actions will have the opportunity to shift to other G.M. plants where we will need more employees to support growth in trucks, crossovers and S.U.V.s,” the automaker said Tuesday — in other words, not the models it has been making in those plants. Ford Motor Company has also cut jobs and dropped sedans from its North American lineup this year.

In addition, G.M. is working on a dozen or more electric vehicles that it plans to roll out in two to four years. Those models will have to be made somewhere, and some could end up in one or more of the three idled plants.