Could President Trump really slap tariffs on Mexican imports – a move that among other things might trigger retaliation against U.S. corn exports?

The man at the helm of U.S. trade policy under the Obama administration isn’t sure.

“It would be big for grain,” says Michael Froman, who until January was the U.S. Trade Representative. “We don’t know, we’ll have to see how it all plays out.”

“Wait and see,” Froman shrugged, after speaking at a forum on trade Tuesday night at the University of Chicago.

Trump vows to use tariffs or a border tax to pay for a wall between the two countries. Once taking office he quickly signed an executive order ending U.S. participation in the Trans-Pacific Partnership and promised to renegotiate the North American Free Trade Agreement. Mexico has already started the process to make that a reality.

But Froman argued that the TPP was already a renegotiation of NAFTA, because Canada and Mexico were included in the deal that addressed issues like currency manipulation and eliminated some tariffs.

Any changes to NAFTA can’t be a one-sided deal that gives the U.S. everything it wants, he said. Mexico and Canada must get something in return for adopting policies the U.S. favors. One reason countries like Mexico signed on to the TPP was because it gave them access to other markets like Japan they’d been shut out of.

“It turns out that other countries have politics too,” said Froman. U.S.-Mexico relations are a staple in debates south of the border, he said. “If you’re going to renegotiate NAFTA, it’s going to have to have to be something they can sell as well. We don’t know yet what the Trump administration means by renegotiating NAFTA.”

He noted the trade pact signed in 1992 had a big impact on agriculture in both the U.S. and Mexico, one that forced some growers there off the farm. The U.S. exported $17.8 billion in in farm products to Mexico in 2016, while the country’s share of U.S. ag exports rose from 6.5% in 1990 to 13.2% last year. Mexico became the biggest customer for U.S. corn growers, importing 548 million bushels last year alone, a quarter of all U.S. sales. At the same time Mexican ag exports to the U.S. went from $2.6 billion in 1990 to $23 billion in 2016.

Froman cited three basic risks from raising barriers to imports. First, countries like Mexico could retaliate against U.S. exports. Unilaterally ending trade agreements also sends a signal “that international obligations don’t matter. Well good luck trying to get China or other countries to uphold their international obligations.”

The final risk is the threat of higher prices for U.S. consumers. “When you raise tariffs on imports you are imposing a tax on the Americans least able to afford it. Low-income Americans spend a disproportionate amount of their income on tradable goods.”

Forcing companies to build more goods here, Froman said, “sounds superficially attractive. Now wait for China to say, Boeing, if you want to sell airplanes in China, you have to shut down in Chicago, you have to shut down in the state of Washington, you have to move your production to China.”

Asked why business leaders have been mostly silent on the trade issue so far, Froman said they focused in the short run on the benefits of tax cuts and regulatory relief promised by Trump. But the president’s order banning immigrants from some countries started to trigger more comments from businesses that could be a turning point, especially because some are being forced to cancel contracts and investment due to uncertainty.

“That is ultimately going to mobilize them to say, you can’t throw away 70 years of a rules-based trading system,” he said. “Or throw away a generation’s worth of immigration policy?”

Workers and communities affected by the changing economy need help, he said, including training and education. But most of the job losses created by change came from automation, not globalization, Froman said.

And imports are not the problem alone in creating a large U.S. trade deficit, he said. ‘We have a terrible trade deficit in coffee,” he joked. “It’s not difficult to run a trade surplus. We had a wonderful trade surplus during the Great Depression.”