MEXICO CITY • The United States filed its first labor complaint with Mexico Wednesday under the U.s.-mexico-canada trade agreement, after an old-guard union was caught allegedly destroying ballots at a General Motors plant in northern Mexico.

The U.S. Trade Representative invoked the “rapid response” mechanism under the trade pact, known as the USMCA.

The mechanism allows a panel to determine whether Mexico is enforcing labor laws that allow workers to choose their union and vote on contracts and union leadership. If Mexico is found not to be enforcing its laws, sanctions could be invoked, including prohibiting some products from entering the United States.

“Using USMCA to help protect freedom of association and collective bargaining rights in Mexico helps workers both at home and in Mexico, by stopping a race to the bottom,” said U.S. Trade Representative Katherine Tai.

Mexican auto workers make one-eighth to one-tenth of the wages of their U.S. counterparts, something that has spurred a relocation of auto plants to Mexico and a loss of U.S. jobs.

For decades, corrupt Mexican unions signed low-wage “protection contracts” behind workers’ backs, often before plants were even opened. Union votes were held by show of hands, or not at all. Workers at many factories in Mexico were unaware they even had a union until they saw dues deducted from their paychecks.

As part of efforts to get the USMCA, which replaced the North American Free Trade Agreement, Mexico passed labor law reform stating all union votes would be by secret ballot, and workers at all factories in Mexico could vote on whether to keep their current union.

It was one such vote among the 6,494 employees of GM transmission and pickup plants in the northern Mexico city of Silao in April that triggered the complaint.