Ohio-based trade lawyer Dan Ujczo said the bilateral U.S. -Mexico talks are necessary because the two countries' one-on-one issues are more significant at this point in the year-long negotiations - but he added that leaving Canada on the sidelines has an added effect.
To be sure, the US and Mexico are close to hammering out their differences on cars, which would allow Canada to rejoin the talks. "Some really good people within both the new and old government, and all working closely together...."
The U.S. and Mexico in recent weeks have made progress on the thorny issue of vehicle manufacturing, as the Trump administration pushes for a deal that would boost factory jobs in America.
But trade experts say Canada's absence is - at least in part - a tactic by the White House to divide and conquer as it applies extra pressure on Ottawa to accept what could be a less palatable deal.
The "sunset clause" - an automatic cancellation of all agreements after five years - has been a major demand from the US administration, but Mexico believes it can agree to a renegotiated version of this.
The sunset issue is one of the biggest remaining obstacles to a Nafta deal as negotiators race to try to land a new Nafta agreement in principle by the end of August.
Donald Trump on Saturday hinted that the U.S. and Mexico were on the verge of reaching an agreement in the NAFTA negotiations that are expected to go on through the weekend, but there are a number of issues that have yet to be resolved.
Asked about a potential deal on Monday, he said "That's why we're here; to make our best effort".
NAFTA negotiators, who began their work one year ago, have missed a series of informal deadlines, from an original goal for a deal by last December to a revised objective for an agreement in the first half of this year.
"We're not negotiating with Canada right now", the president said during a televised cabinet meeting. Lighthizer and Jared Kushner, Trump's son-in-law and adviser, declined to comment on Sunday afternoon as they left the trade representative's offices.
Obrador has told Trump that he would like the deal completed before he takes office.
The team of leftist Mexican president-elect Andres Manuel Lopez Obrador has resisted enshrining the 2013-14 opening of the oil and gas sector enacted by outgoing president Enrique Pena Nieto in the new NAFTA, people close to the talks say. Jesus Seade, the NAFTA representative for Mexican president-elect Andres Manuel Lopez Obrador, on Saturday predicted that the nations will agree on a lighter version of a so-called sunset clause, an automatic expiration after five years - a key US demand.