After a monthlong strike that has idled General Motors plants across the Midwest and South, union leaders are gathering on Thursday in Detroit to consider a deal that could send workers back to the assembly lines.

But even before the meeting started, there were signs of dissent from union members gathered outside the Renaissance Center office complex, where G.M. is headquartered and the contract negotiations took place. As the roughly 200 union officials who will vote on the proposed contract arrived for the meeting, they were greeted by about 30 workers from the Lordstown, Ohio, plant in red T-shirts shouting, “Vote no!”

G.M. shuttered the Lordstown plant earlier this year as it discontinued production of the Chevrolet Cruze. An electric-vehicle maker is supposed to take over the factory, and G.M. has proposed a joint venture to build a battery plant nearby that would hire union workers under a separate contract.

While the details of the tentative contract agreement have not been publicly announced, there is no indication that G.M.’s reactivation of the Lordstown plant was ever on the table.