Trade has become a target this presidential campaign season. Both Democrats and Republicans have been attacking trade agreements as “unfair” to American workers.That resonates in places like Massena, N.Y., where voters cast primary ballots this week.

For more than a century, the Alcoa company has been making aluminum in Massena, a small city on the banks of the St. Lawrence River. Workers at Alcoa plants there made good wages — $20 to $30 an hour. But two years ago, faced with falling prices, the company decided to shutter one of its plants, eliminating more than 300 jobs.

“This influx of metal into the market from China and other countries, it’s flooded the market and drove the price down,” said Bob Smith, president of the local steelworkers union. “It’s hard to compete against that. That’s why we feel it’s unfair.”…