"Mr. President, America's corn farmers helped elect you," Wesley Spurlock of the National Corn Growers Association warned in a statement. "Withdrawing from NAFTA would be disastrous for American agriculture."
Within hours, Trump softened his stance. He wouldn't actually dump NAFTA, he said. He'd first try to forge a more advantageous deal with Mexico and Canada — a move that formally began Thursday when his top trade negotiator, Robert Lighthizer, informed Congress of the administration's intent to renegotiate NAFTA.
As a candidate, Trump defined his "America First" stance as a means to fight unfair foreign competition. He blamed unjust deals for swelling U.S. trade gaps and stealing factory jobs.
But NAFTA and other deals have been good for American farmers, who stand to lose if Trump ditches the pact or ignites a trade war. The United States has enjoyed a trade surplus in farm products since at least 1967, government data show. Last year, farm exports exceeded imports by $20.5 billion.
''You don't start off trade negotiations ... by picking fights with your trade partners that are completely unnecessary," says Aaron Lehman, a fifth-generation Iowa farmer who produces corn, soybeans, oats and hay.
Many farmers worry that Trump's policies will jeopardize their exports just as they face weaker crop and livestock prices.
"It comes up pretty quickly in conversation," says Blake Hurst, a corn and soybean farmer in northwestern Missouri's Atchison County.
That county's voters backed Trump more than 3-to-1 in the election but now feel "it would be better if the rhetoric (on trade) was a little less strident," says Hurst, president of the Missouri Farm Bureau.
Trump's main argument against NAFTA and other pacts was that they exposed American workers to unequal competition with low-wage workers in countries like Mexico and China.
NAFTA did lead some American manufacturers to move factories and jobs to Mexico. But since it took effect in 1994 and eased tariffs, annual farm exports to Mexico have jumped nearly five-fold to about $18 billion. Mexico is the No. 3 market for U.S. agriculture, notably corn, soybeans and pork.
"The trade agreements that we've had have been very beneficial," says Stephen Censky, CEO of the American Soybean Association. "We need to take care not to blow the significant gains that agriculture has won."
The U.S. has run a surplus in farm trade with Mexico for 20 of the 23 years since NAFTA took effect. Still, the surpluses with Mexico became deficits in 2015 and 2016 as global livestock and grain prices plummeted and shrank the value of American exports, notes Joseph Glauber of the International Food Policy Research Institute.
Mexico has begun to seek alternatives to U.S. food because, as its agriculture secretary, Jose Calzada Rovirosa, said in March, Trump's remarks on trade "have injected uncertainty" into the agriculture business.
Once word had surfaced that Trump was considering pulling out of NAFTA, Sonny Perdue, two days into his job as the president's agriculture secretary, hastened to the White House with a map showing areas that would be hurt most by a pullout, overlapped with many that voted for Trump.