GM hasn't assigned new vehicles to the plants and hourly union workers aren’t scheduled to begin being laid off until March 1, months after the “holiday season.”
The letter writing campaign comes nearly three weeks after GM announced the moves, which could impact roughly 6,700 U.S. plant workers, including 800 salaried, 3,300 UAW hourly workers at assembly plants in Detroit-Hamtramck, Mich., and Lordstown, Ohio and powertrain factories in Warren, Mich. and White Marsh, Md.
“Right now, workers and their families need us to stand in solidarity with them against this callous decision,” the UAW said Friday. “Without job security or stability, many of our UAW brothers and sisters are headed into the new year unsure of how they’ll provide for their families or earn a living.”
With normal attrition rates, a GM spokesman said the company is confident that all affected hourly workers will be eligible for another job if willing to move to another plant.
GM said many salaried employees at plants losing production "will have opportunities at other GM locations."
The formal layoff notices will begin going to government agencies on Friday and will continue into 2019, GM said.
GM CEO Mary Barra, who came under fire from lawmakers for how the automaker disclosed the job cuts last month, said in a statement Friday that GM's "focus remains on providing interested employees options to transition including job opportunities at other GM plants."
GM said in November it will cease production at five North American plants -- including three assembly plants in Ohio, Michigan and Ontario -- next year and cut up to 15,000 jobs. GM is ending production of a half dozen cars, citing slow-selling sedans as one reason for the need to restructure.
A big chunk of the job cuts include plans to trim 15 percent of GM's North American salaried workforce by early next year.
GM said salaried workers who are losing their jobs are receiving "out-placement services including job search assistance, career counseling, resume writing and interview skills."
GM, which announced in November it is ending production at its Oshawa assembly plant in Canada, also said it is working with "dealers, local colleges and other employers to train and help secure jobs for impacted workers from the Oshawa Assembly Plant who are interested." Canadian union Unifor has vowed to fight the plant closure.
Two U.S. senators earlier asked GM other questions on the layoff impacts, including how many jobs will be cut at suppliers as a result.
Michael Wayland contributed to this report.