Political considerations in all three countries are playing a role in the timeline to complete a revised Nafta.

The president-elect of Mexico, Andrés Manuel López Obrador, takes office on Dec. 1, and its current president, Enrique Peña Nieto, wants to sign the new Nafta agreement as a final act before he departs. Mr. Lighthizer said that the fate of the agreement under the next Mexican administration is uncertain and that he does not want to have to restart talks with Mexico’s new leadership.

Canada’s internal politics are also a factor given provincial elections on Oct. 1. The Canadian government may be reluctant to finalize any agreement that contains dairy concessions ahead of a vote in Quebec, whose economy is heavily dependent on dairy, trade analysts said.

The Trump administration wants the current, Republican-controlled Congress to ratify a new deal by Dec. 1. Since Mr. Trump notified Congress of the administration’s deal with Mexico last month, the administration now has until Sept. 30 to release text of a new trade agreement.

Some on Capitol Hill say there may be some wiggle room on the end-of-September deadline. Congress could take several routes to allow the administration to submit a final agreement in October or later, but still allow Mr. Trump to sign the deal in November. Several congressional staff members stressed in interviews this month that the deadline is only binding if Congress chooses to enforce it — and that it may choose not to.

Under United States trade law, the final text of the trade agreement must be public for 60 days before it is signed. Even if that happens before the next Mexican administration takes office, there likely will still be additional obstacles in Congress, which has the final say on trade treaties. A vote is unlikely to occur until next year and this could be complicated if Democrats take control of the House or the Senate.

Ms. Freeland, who has been engaged in marathon talks with Mr. Lighthizer in Washington in recent weeks, demurred on Tuesday when asked about the specifics of the differences between the two countries. She emphasized her hope of reaching an agreement and underscored that doing so is also important to the United States because Canada is its largest market.