(Bloomberg) -- Workers at a giant General Motors Co. truck plant in Mexico will choose their union in a key vote that will signal whether North American free trade rules are improving labor rights in the country.

All workers eligible to be unionized at the plant are being called to vote on Feb. 1 and 2 among four unions for whichever they prefer to represent them in the negotiations regarding a new collective contract. 

In August, workers at the plant voted to cancel their union contract after the U.S. initiated a dispute against conditions at the factory, in what represented a historic victory for the new North American free trade agreement, known as USMCA. The agreement sets out labor provisions for Mexico that are meant to drive out unions that for decades haven’t represented their workers.

Read More: GM Mexico Plant Rejects Union in Historic U.S.-Backed Vote

A previous vote at the GM plant in April was thrown out after Mexico found irregularities in the election held by the union, a chapter of Mexico’s largest and most entrenched syndicate, known as CTM. That union will be on the ballot this time around, according to a statement by Mexico’s labor board.

Secret Ballots

GM will “continue to collaborate with authorities so this democratic exercise is carried out successfully,” a company spokesperson said Friday. “It’s very important for the company that workers exercise their rights.”

As part of the revamped North American trade deal, Mexico President Andres Manuel Lopez Obrador’s administration pushed through a law in 2019 that requires unions to hold votes by secret ballot to validate their labor contracts. 

Mexico’s government estimates that before the law was passed about 80% of union contracts were signed without the knowledge of employees and grant them little more than their basic legal rights. These so-called protection contracts have helped keep wages low and companies happy in Mexico.

The GM plant has more than 6,000 unionized workers and is located about 215 miles (350km) northwest of Mexico City.

Read More: U.S. Asks Mexico to Probe Labor Rights at GM Truck Plant

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