Boeing Co.’s Kelly Ortberg urged members of the company’s largest union not to strike, cautioning a work stoppage would harm efforts to turn around the planemaker’s factories after years of turmoil.

The risk of a strike at Boeing appears to be growing, as factory workers complain about a contract offer that their union negotiated with the giant aircraft manufacturer.

The head of a labor union representing 32,000 workers at Boeing (NYSE:BA) said he expects rank-and-file members will reject a proposed contract and vote to strike. “The response from people is it’s not good enough,