MEXICO CITY — Workers at a General Motors plant in northern Mexico began voting Tuesday on whether to form one of the first truly independent auto labor unions in Mexican history.

The vote among the roughly 6,500 employees of GM transmission and pickup plants in the city of Silao is a major test of whether a measure of freedom has come to Mexican labor practices.

For almost a century, Mexican unions have been largely a sham, with sold-out leaders guaranteeing low wages that drained manufacturing jobs out of the United States. Mexican auto workers make one-eighth to one-tenth of the wages of their U.S. counterparts, spurring a massive relocation of auto plants to Mexico.

Under changes to Mexican labor law required under the U.S.-Mexico-Canada free trade pact, workers can now in theory vote out the old, pro-company union bosses.

But independent labor activists still face threats and pressure tactics; a union activist said that just two days before the voting began, thugs told her not to show up for the vote.

“They just came by my house, two men and a woman, telling me to send a statement saying neither I nor any other worker should show up tomorrow, or if not there will be problems,” said Alejandra Morales Reynoso, the leader of the Independent Union of Auto Industry Workers, known by its initials in Spanish as Sinttia.

“That is what we call a direct threat against me, my family and my coworkers here,” Morales Reynoso said Monday.

United Automobile Workers President Ray Curry called on Mexican labor authorities and General Motors to “allow for the presence of international and domestic observers to ensure no threats or intimidation occurs.” Mexico’s electoral institute and human rights agency said they are sending observers.

In the ballot vote Tuesday and Wednesday, plant employees can choose which union will represent them: Sinttia, or one of three old-guard unions.