On April 5, the same day that Mr. Trump’s negotiators were meeting in Washington with their Chinese counterparts to haggle over their trade deal, Mr. Trump threatened to put a 25 percent tariff on Mexican cars “if for any reason Mexico stops apprehending and bringing the illegals back to where they came from.”

“This will supersede USMCA,” the president wrote.

It is not clear that the president, who has repeatedly threatened auto tariffs on trading partners, would follow through with his threat. If he does, Canada and Mexico could reasonably argue that all of the agreements reached in the U.S.M.C.A. are void, resulting in the breakup of a trade pact, which is a critical agreement for businesses across North America.

“Because he is so unpredictable, you are not sure he’ll stick to anything,” said Maryscott Greenwood, chief executive of the Canadian American Business Council.