President Trump said on Monday that the United States and Mexico had reached an accord to revise key portions of the North American Free Trade Agreement and would finalize it within days, suggesting he was ready to jettison Canada from the trilateral trade pact if the country did not get on board quickly.

Speaking from the Oval Office, Mr. Trump promoted the preliminary agreement with Mexico as a deal that could replace Nafta and threatened to hit Canada with auto tariffs if it did not “negotiate fairly.”

“They used to call it Nafta,” Mr. Trump said. “We’re going to call it the United States-Mexico Trade Agreement,” adding that the term Nafta — which he has called the “worst” trade deal in history — had “a bad connotation” for the United States.

Yet while Mr. Trump may try to change the name, the agreement reached with Mexico is simply a revised Nafta, with updates to provisions surrounding the digital economy, automobiles, agriculture and labor unions. The core of the trade pact — which allows American companies to operate in Mexico and Canada without tariffs — remains intact.