“I will be formally terminating Nafta shortly,” Mr. Trump told reporters aboard Air Force One en route from the Group of 20 conference in Buenos Aires, a day after appearing at a ceremonial signing of the new deal with Canada and Mexico.

If the president follows through on his threat, congressional leaders will have six months to pass the measure. The agreement has been losing support in recent days as Democratic lawmakers, ready to take control of the House in January, reckon with fallout from the announcement last week that General Motors was planning to idle five plants in North America.

If no deal can be reached, both versions of the treaty would be void, which would result in far more restrictive trade that could have a severe impact on industry and agriculture in all three nations, economists have warned.