While the European Union is Canada’s second-biggest trading partner, Wallonia, with 3.5 million people, represents only a tiny fraction of the bloc’s 508 million inhabitants. Canada accounts for just 0.20 percent of Wallonia’strade. But Wallonia’s economy has stagnated in recent decades, as its steel mills have closed down, like those of the American Rust Belt and of northern England; the region has lost ground to Dutch-speaking Flanders.

The stagnation has opened the door to trade skeptics like Mr. Wiertz, who argue that the deck is stacked against them.

Global competition has squeezed small businesses dry, said Mr. Wiertz, a fifth-generation farmer.

“We now produce milk at the world price,” he said. “Only big multinationals can afford to do that, but they don’t pay their fair share of taxes, or they take their capital abroad.”

In 2013, when milk prices dropped for the fourth year in a row, Mr. Wiertz and Mr. Hollands started a movement against free-trade agreements and austerity policies. With help from Raf Verbeke, a union activist who manages their public relations campaign, they formed a coalition called D19-20, after the dates of its first protest, Dec. 19 and Dec. 20, 2013.

Today, D19-20 counts more than 90 member organizations, including a 300,000-strong public workers’ union, a 150,000-member student alliance and the European Milk Board, an umbrella organization for dairy farmers in 15 countries. Over time, Mr. Wiertz said, the coalition persuaded left-leaning political parties to take its side. Its efforts culminated with the Wallonian Parliament’s vote to reject the deal two weeks ago — although that decision has now been reversed.

Wallonia, to be sure, does not speak for all of Belgium. Support for D19-20 is stronger in Wallonia than it is in Flanders. Others see opportunity.

“Our sector is desperately looking for new markets,” said Florent Geerdens, who owns and manages a 100-acre farm in Flanders that grows apples, pears and cherries. “Especially since we lost Russia in 2014.”