G.M. workers in Mexico pick an independent union, a trade deal test case.

Workers at one of the largest General Motors plants in Mexico voted to adopt an independent union on Thursday in what was seen as an important test case for whether new North American trade rules can improve working conditions and stamp out corruption in the Mexican labor system.

The union, called the Independent National Autoworkers Union, won 78 percent of the votes cast at the plant in Silao, where more than 6,000 workers assemble Chevy Silverados and G.M.C. Sierra pickup trucks. The vote pushed out the Confederation of Mexican Workers, which had held the contract for the last 25 years.

Workers at the Silao plant start out earning less than $9 a day, and have described punishing working conditions. Employees have said they are often denied breaks and are rarely offered raises.

Mexico pledged to make sweeping changes to its labor laws and court system as part of the new United States Mexico Canada Agreement, which replaced the North American Free Trade Agreement. The new agreement aimed to make it easier for independent unions to challenge incumbents, and required companies operating in Mexico to revisit hundreds of thousands of existing contracts in independent elections.

Democrats hailed those reforms as among the pact’s most important changes, saying they would help level the playing field among workers in the United States, Canada and Mexico.

Labor leaders have hoped the vote at the G.M. plant would presage changes at other factories around Mexico, where existing labor unions have been accused of colluding with company management to keep wages low.

Liz Shuler, the president of the AFL-CIO, said in a statement that the election “set a hard-won precedent and came only after workers voted to throw out a previous contract that had poor benefits and was negotiated without the workers’ input.”

“Workers overcame gross intimidation and election meddling, and their triumph is an example of what happens when workers stand together. This vote represents a rejection of the past and a new era for Mexican workers’ right to associate freely,” she said.

The U.S. secretary of labor, Marty Walsh, issued a statement expressing support for the vote.

“As workers, we are stronger when we can speak with one voice — and we are stronger when our fellow workers around the world can do the same,” he said. “The work of defending freedom of association never stops, but this historic election shows us that we can make progress toward the right of all workers to associate freely when we work together.”