Estimates are that nearly one out of two tomatoes eaten in the United States comes from Mexico — a statistic Florida growers would like to change, even at the risk of a trade war.

On Thursday, they got a reason to hope.

The United States Department of Commerce signaled then that it might be willing to end a 16-year-old agreement between the United States and some Mexican growers that has kept the price of Mexican tomatoes relatively low for American consumers. American tomato growers say the price has been so low that they can barely compete.

Within hours of the American action, Mexico threatened to retaliate, claiming that the Obama administration was trying to placate farmers in an important swing state. The Mexican government has support from seemingly unlikely backers in the United States: the big box stores like Walmart, which fear they will have to raise their prices, and other commodity producers, who worry that their products will be caught in a trade war.

“It will be very unfortunate if this devolves into a shooting war because this becomes a tit-for-tat and in the end, nobody wins,” said John Keeling, chief executive of the National Potato Council.