Mexico has been even more pointed in resisting the assertion that there is a problem. The economy minister, Ildefonso Guajardo Villarreal, told a Mexican Senate commission last week that he was “delighted to analyze the situation that we call ‘trade rebalancing’ if and when we manage to improve that through expanding trade, not restricting it.”

A key question looming over the negotiations is how the Trump administration’s public bombast will translate into the details of the negotiations. The administration in its early months has repeatedly talked tough and then sought to conciliate trading partners.

The administration, for example, insists that it wants to do away with a system of independent arbitration that allows companies to seek the elimination of tariffs. The system has been used primarily by Mexican and Canadian companies to force the United States to abandon protectionist measures found to be in violation of the agreement.

Another area of potential conflict concerns the automobile industry. The United States wants to discourage importation of auto parts from countries outside the Nafta region. Under the current agreement, a car assembled in Mexico can be imported into the United States without paying an import tax if at least 62.5 percent of the car, measured by value, was made in North America. The Trump administration wants to raise that bar, and to require that a significant portion of those parts come from the United States.

The United Automobile Workers union has long sought such a change.

But carmakers are wary. The importation of some cheap parts helps to hold down the cost of the final product. In general, a higher share of Nafta components, and a higher share of American components, means a more expensive car.

“Many in the business community feel that the Nafta is working quite well and they don’t want disruption in existing supply chains,” said Jeffrey J. Schott, a Nafta expert at the Peterson Institute for International Economics in Washington. Both Canada and Mexico said Wednesday that they opposed specific standards for the share of car parts coming from any of the three nations.