President Donald Trump's threat to tear up the North American free trade pact would cause economic suffering in the United States in a decision that would also be terrible politics, Prime Minister Justin Trudeau told students in Chicago Wednesday.

Millions of American workers would be harmed, and their lives disrupted in the short-term through a thickening of the border and greater uncertainty, even if Canada and the United States can finalize a deal down the road, he said.

Trudeau said that Canadians are rightly nervous that NAFTA will be torn up — a repeated threat Trump has made over successive rounds of talks between Canada, the United States and Mexico — and what it would mean for jobs on both sides of the border.

"Even if theoretically there is a better opportunity for a long-term deal, in the short-term that's a lot of families out of work (and) suffering in a way that I think would be far worse politically," Trudeau said during an event at the University of Chicago.

"The challenge we have is not trade deal versus no trade deal. It's how do we make sure we're benefiting citizens and workers who don't feel like they've been properly supported or cared for over the past years."

In the audience was a group of Midwest students and officials, some of whom are skeptical that trade would help them. Trudeau said that ending free trade between Canada and the United States would hurt the wealthy, but also harm future opportunities for the U.S. middle class.

Trudeau argued the case for free trade hadn't been properly made, which has helped fuel the economic anxiety that students and other Americans feel. The key to reducing the grumbling around globalism is making sure that the rules and policies around trade ensure everyone feels the benefits, he said.

Trudeau framed the North American Free Trade Agreement as being good for Canada and the United States, but was clear that Canada wouldn't be bullied into signing. He said he had concerns about an American proposal for a five-year sunset clause that would cause uncertainty and stifle investment.

"We know we can work towards a good deal, but we also know that we will not be pushed into accepting any old deal," Trudeau said. "No deal might very well be better for Canada than a bad deal. And being firm on that is, I think, what Canadians expect of me."

Canada, Illinois trade

Trudeau left the stage to a standing ovation from the auditorium almost filled to its capacity of about 1,000, wading through the crowd of students and shaking the occasional hand as The Tragically Hip's "Courage" played over the loudspeakers.

Although a "blue" state and city that may be sympathetic to his talk about trade and the environment, Illinois and Chicago were anything but an easy sell for Trudeau on the first day of a four-day swing through the United States.

A small group of protesters quietly gathered outside Trudeau's event at the University of Chicago, demanding he follow through on a pledge for stronger labour and environmental provisions in a new NAFTA.

Union leaders say Illinois has lost 290,000 manufacturing jobs since NAFTA went into effect, while the Illinois Chamber of Commerce says the state has benefited enormously from free trade.

The trade relationship between Illinois and Canada came up repeatedly during public events Trudeau had earlier in the day, first with Illinois Gov. Bruce Rauner and then Chicago Mayor Rahm Emanuel.

The TPP

Rauner and others spoke of how Canada is the state's top trading partner, that many large Canadian companies employ thousands of people in the state, and that Canadian oil flows to refineries in the region.

"It's easy to forget about those things if you're not reminded," said Todd Maisch, president and CEO of the Illinois Chamber of Commerce, which promotes local jobs and economic opportunity.

During a question-and-answer session with students at the University of Chicago's Institute of Politics, Trudeau talked about the trade deal Canada has signed with Pacific Rim countries that don't include the United States.

That kind of talk will get more attention from the anti-trade crowd, Maisch said.

In San Francisco, Trudeau will meet Amazon's top executive, Jeff Bezos, among other tech sector executives.

In Los Angeles, Trudeau will deliver a speech Friday about the merits of free trade to local, state and congressional officials at the Ronald Reagan Presidential Library and Institute.