A new and independent union won the right to represent workers at General Motors Co.’s truck plant in Mexico—a sign that new North American free trade rules are bolstering the nation’s labor rights.

The union, known as SINTTIA, won the vote by a wide margin, Mexican labor authorities said. The group defeated the CTM, Mexico’s entrenched labor group, to represent more than 6,000 workers in labor negotiations at the plant, which produces the Chevrolet Silverado and GMC Sierra models in Silao, a city in central Mexico.

The victory by an independent union could start to break the CTM’s longstanding hold on Mexican labor wages and begin the long process of bringing pay closer to what workers make in the U.S. and Canada. Workers and labor rights groups have criticized the CTM for years, alleging that it negotiated low-wage “protection contracts” that suppressed pay for the very workers the union was supposed to represent. A Bloomberg investigation substantiated the claims.

Workers at the Silao plant will now prepare to bargain for a new contract with GM. Employees there say they earn less than $25 a day, compared to a range of $18 to $32 per hour at GM plants in the U.S. and Canada. Mexico’s low wages—and its unions that traditionally look out for employers’ interests ahead of workers’—have been targeted in the USMCA, as the regional trade pact is known.

“SINTTIA is looking forward to bargaining a decent collective agreement,” said Mohamad Alsadi, director of the international department for Unifor, Canada’s autoworkers union. Unifor has been helping SINTTIA with its organizing drive.

Workers overwhelmingly voted for SINTTIA, which got 77% of the 5,478 votes.

Before the election, the largest unions in the U.S. and Canada said there were “substantial reasons to doubt” the process would be free and fair. Canada’s Unifor said CTM officials engaged in vote-buying, while SINTTIA’s head said three people visited her home to threaten her safety if she showed up at the vote.

Mexican and U.S. authorities, as well as independent observers, closely monitored the vote on Feb. 1 and 2.

Eligible workers at the plant voted among four unions. The winning group will engage in negotiations with GM on a new collective contract.

In August, workers at the plant voted to cancel their union contract after the U.S. initiated a dispute over conditions at the factory—a first under the new trade regime known as the USMCA. The agreement set labor provisions for Mexico that are meant to drive out unions that for decades have favored the interests of employers over those of workers.

A previous vote at the GM plant in April was thrown out after Mexico found irregularities in the election, which was held by a chapter of the CTM.

As part of the revamped North American trade deal, Mexico President Andres Manuel Lopez Obrador’s administration pushed through a law in 2019 that expands workers’ rights, including union votes by secret ballot to validate labor contracts.

Mexico’s government estimates that before the law was passed, about 80% of union contracts were signed without the knowledge of employees. These so-called protection contracts, which grant little more than basic legal rights, have helped suppress Mexican wages and give employers more control over their workforce.