A recent article in The New York Times took me back to the latter decades of the 20th century and the debates over free trade agreements. The 1988 federal election was fought over the issue and the free-trade side led by Prime Minister Brian Mulroney won the day. A free trade agreement with the U.S. came into effect on January 1, 1989. Five years later the agreement was extended to Mexico as the North American Free Trade Agreement (NAFTA).

Supporters of the deal insisted it was Canada’s best chance of succeeding in the increasingly competitive global market. It would create jobs, attract foreign investment, and provide access to American consumers. I remember Thomas d’Aquino, CEO of the Canadian Council of Chief Executives, reassuring us that what was good for the corporate sector was good for all of us—the infamous trickle down theory.

Opponents saw a loss of jobs and sovereignty. They included trade unionists, teachers, environmentalists, women’s and Indigenous groups, and various other social and cultural movements. Another opponent, not yet in the political arena, was Donald Trump.

The Times article was about two Americans who shared the opponents’ skepticism. One was a liberal activist with a consumer advocacy organization; the other a conservative trade lawyer. On opposite ends of the political spectrum they nonetheless agreed on one thing: NAFTA would hurt working people. They worried that the economic interests of the working class would be sacrificed to the profit of corporations. They were, of course, proven right. They also lobbied against the World Trade Organization and, specifically, China’s admission.

All these years later the two remain friends, differing in their politics but still allies on free trade.

The activist, Lori Wallach, is now director of the ReThink Trade program at the American Economic Liberties Project, a nonprofit research and advocacy group. The conservative lawyer, Robert Lighthizer, has enjoyed an opportunity to put some of his beliefs into action. Donald Trump made him U.S. trade representative. You may remember him from the tough renegotiating of NAFTA, now CUSMA.

Lighthizer was the implementer of President Trump’s economic populism. In addition to renegotiating NAFTA, he hit China with tariffs and froze the World Trade Organization’s appeals court by refusing to nominate new judges.

He did all this with the help of Ms. Wallach and other progressive Democrats while Wall Street and corporate-oriented Republicans howled. In his book No Trade Is Free, Lighthizer singled out Ms. Wallach as “a longtime friend and co-conspirator.”

What they accomplished is not going away. The Biden administration has kept and even expanded the practices. Biden’s U.S. trade representative talks of trade policies being cantered on the needs of American workers, rather than on multinational corporations.

In 2016, while Clinton partied with bankers and Obama focussed on yet another free trade agreement, Trump kept his finger on the pulse of working class Americans. He knew how they felt about free trade because that’s how he felt about it. Demagogues often excel at detecting, and sharing, the fears and prejudices of the masses and exploiting them for political gain. Indeed, it’s what they are all about and Trump has proven to be very good at it.

The lesson for the Democrats, the lesson for progressives generally, is not to be distracted from what your major focus should be: the interests, particularly the economic interests, of the working class.

While Ms. Wallach is proud of what she and Lighthizer accomplished under Trump, she has found it hard to accept his continued loyalty to the former president, to a man she believes has instigated an “all but unprecedented threat to U.S. democracy.” If more Democrats had kept their eye on the ball as she did, Trump might never have become a problem.