It's unlikely he would want to rewrite the whole deal, said Schott, who's also a member of two government advisory committees on trade.
"If we push it beyond that date, then we have a new negotiation with Lopez Obrador and we don't know where that would go at all", Lighthizer said.
The US and Canada are under pressure to reach a deal by September 30.
Throughout most of the talks, Pena Nieto's administration had reasoned that, together, Canada and Mexico had more leverage in negotiations that were stacked in Trump's favor. The president said it would be called "USM", for the US and Mexico, instead of "USMC", and offered blunt criticism of the Canadian team engaged in the talks.
The attack cast further doubt on the future of the three-nation North American Free Trade Agreement, which underpins $1.2tn in annual trade between Canada, Mexico and the United States.
He added that the United States was more than willing to slap tariffs on Canadian auto imports, if Canada didn't acquiesce to his demands to reduce tariffs on USA dairy.
The United States will likely retain more auto-industry jobs after Mexico agreed to impose minimum salary requirements on the sector - at least $16 an hour for between 40 and 45 percent of workers, five times what most earn now.
On NAFTA, the USA and Canada remain in "near-continuous negotiations", Trudeau said, adding that his foreign minister - who is leading the NAFTA file - will address the United Nations on Saturday, signaling that there won't be a last-minute bargaining push in Washington that day. "We're going to tax the cars that come in", he said.
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U.S. President Donald Trump says he rejected a one-on-one meeting with Prime Minister Justin Trudeau because he's upset with Canadian tariffs - but Trudeau's office says it never extended an invitation.
Speaking separately, Canada's ambassador to Washington said that on a scale of 1 to 10, the chances of an agreement by the September 30 deadline were 5.
"We are not getting along with their negotiators", he said.
"I mean Mexico has 25 percent of our oil business now because of NAFTA", said Trump. If Canada comes along now, that would be the best. He's promised to rip up NAFTA, which he called "the worst deal maybe ever signed".
Trump has said "a few times" that a new deal would mitigate the tariff issue, the prime minister said.
"Those days are over", said Trump, who didn't specifically mention Canada in his speech.
The U.S. imposed tariffs on imported steel and aluminum from some countries in March and added Canada, Mexico and the European Union in June.
"The United States moving ahead without Canada shows that Canada may be left out altogether", said Adam Button, a currency analyst at ForexLive. "It's been long and exhausting, but I think we've narrowed the gap".