MEXICO CITY (AP) — Workers at a General Motors plant in northern Mexico were 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 northern Mexico 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; just two days before the voting began, thugs threatened a union activist and 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 secret-ballot vote Tuesday and Wednesday, plant employees can choose which union will represent them: Sinttia, or one of three old-guard unions, including one from the long-dominant Confederation of Mexican Workers, the CTM. Results were expected to be announced Thursday.

The CTM actually formed part of Mexico's old ruling party, and its leaders would sign union contracts with companies before they even opened plants. They were so secretive many workers didn't even know they had a union until they saw dues deducted from their paychecks.

The old-style union bosses were ruthless in threatening or allowing companies to fire dissident workers and often decided union votes with thugs, show-of-hand votes or gunplay.

One such CTM union had been in power at the GM plant in Silao, and when workers voted on whether to oust it in April, Interior Department inspectors "discovered that at the offices where the union itself had the ballot boxes, ballots that were part of the vote had been destroyed, making it impossible to continue with the vote."

The violations were so blatant that the U.S. government filed a labor complaint under the USMCA. A vote was rescheduled in August, and confirmed the ouster of the old union.

In a statement Tuesday, GM said the company "has been absolutely committed to working with the Mexican authorities, the workforce, vote observers and all partners including the Administration and U.S. Congress to provide the environment for a free and fair election by the workers of Silao. This includes reporting for investigation any allegations of harassment or intimidation. We look forward to working with whichever union leadership is selected by our workforce."

But it has been a long, painful road. Israel Cervantes, a fired GM worker fighting for reinstatement, says about 20 workers have been fired because of union activism since the battle began in early 2019.

"Mainly, we think they (the threats) are coming from somebody, some of the unions that are running" in this week's vote, Cervantes said. The CTM has not publicly commented on the dispute.

While this week's vote is good, the Silao plant appears to be an exception to the rule, said lawyer Susana Prieto, Mexico's most outspoken labor activist, who is now serving in Congress for the ruling Morena party.

"Yes it is an achievement; it's an achievement because of the intervention by U.S. government authorities," Prieto said, adding Mexican officials "were forced to do it" and saying "it is the only case in the country."

Prieto said she also doubted the U.S. government was all that committed to union democracy in Mexico, or wouldn't be willing to exchange it for something it wants from Mexico, like acceptance of proposed subsidies for U.S.-made electric vehicles.

"It is a bargaining chip," Prieto said.

"You may be proud of the U.S. pressure in the no-confidence vote at General Motors," Prieto said. "But I am very disappointed with the U.S. government and the way it backed down on the question of Tridonex," a border assembly plant where a company accused of harassing and firing workers to block an independent union drive was permitted to settle a complaint simply by promising to allow future organizing and give severance pay to some fired workers.

Prieto said a full-scale USMCA complaint and an ouster vote would have been better than a negotiated solution that was based on company promises and the sacking of the dissident union activists.

Prieto said that while Mexico's new laws may look good on paper, they are vulnerable to the same old practices, even after old-guard unions lose no-confidence votes.

She said that in border assembly plants in the state of Tamaulipas, old-guard unions are have been allowed to coerce workers even after being voted out. Many simply run again in subsequent votes meant to replace them.

"When they lose, the union delegates and the union secretary general come in with CTM officials and they say, 'The farce is over. Which union are you going to belong to?'" Prieto said. "'This job is going to be eliminated,' and so out of fear of losing their jobs they (workers) say, 'I'll go with you.'"