Mexican salaries are low, partly because of unions that collude with employers. Workers at a G.M. factory are challenging that system, testing the labor reforms in the reworked North American trade deal.

When he got a job with General Motors in Mexico, Guillermo Ramírez thought it was his ticket out of poverty.

But a decade later, Mr. Ramírez says he still doesn’t earn enough to care for his three children. They eat at his mother’s house, while he skips meals and borrows a car to take his 7-month-old baby, who suffers from seizures, to the hospital.

“You’re earning so little,” said Mr. Ramírez. “It makes you feel useless.”

Mexico has transformed into an industrial powerhouse over the last two decades, attracting a torrent of investment from some of the world’s largest companies. And yet, a stubborn problem persists: Though the country has become one of the richest in Latin America, its workers still earn among the lowest salaries of almost any nation in the region.