(Bloomberg) -- The largest unions in the U.S. and Canada are raising questions over the fairness of a vote that’s set to take place at General Motors Co.’s truck plant in Mexico. 

Workers eligible to be unionized at the plant have been asked to vote Feb. 1 and Feb. 2 on which of four unions they prefer to represent them in negotiations for a new contract.

Canada’s Unifor union said in a letter to Mexican labor authorities Jan. 18 that there are “substantial reasons to doubt” the vote will be free and fair. In a separate statement, the American Federation of Labor & Congress of Industrial Organizations, known as the AFL-CIO, said it’s “concerned by the lack of protection for worker’s rights inside the GM plant.”

One of the unions that will be on the ballot in two weeks is a chapter of Mexico’s largest and most entrenched syndicate, known as CTM. Unifor said the CTM union has used previous delays in the vote to campaign in the workplace and put in place additional unions to divide worker votes. “One of these unions appeared suddenly, with no history of representation or membership in the plant,” Unifor said.

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

Read More: GM Workers at Mexico Truck Plant To Hold Landmark Union Vote

The AFL-CIO said that GM and the Mexican labor authority need to guarantee the conditions for a fair and open union election “as prescribed by the USMCA and the Mexican labor reform.”

GM didn’t immediately reply to a request for comment. Unifor and the AFL-CIO didn’t return calls for added comment. The Labor Ministry had no comment.

A new union formed by GM workers after they canceled their labor contract said the ballot was being padded by little-known syndicates that would fracture the vote. The union, known as Sindicato Independiente Nacional de Trabajadores y Trabajadoras de la Industrial Automotriz (SINTTIA), called for authorities to respect union freedom in a Jan. 18 statement.

Hugo Varela, the CTM confederation leader in Guanajuato, said his affiliate collected the signatures needed to be on the ballot in a fair manner. “This process has to be very transparent and clean and respect the will of the people.”

The other two unions on the ballot couldn’t be reached for comment.

As part of the revamped North American trade deal, Mexican 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.

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

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

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