Trump was on board Air Force One on his way back from Buenos Aires when he declared he would terminate NAFTA, a move that gives lawmakers six months to approve its replacement. The USMCA requires at least 40 percent of auto production to come from factories with an average wage of 16 US dollars per hour.
Pelosi, who is expected to take the role of House Speaker in January, has repeatedly expressed deep concerns over the deal's enforceability.
In an interview with CNN on Sunday, he said, "We can go back to the table with the Mexicans and the Canadians and do stronger labor standards". "I will only support this new deal if we can make it a better deal for Wisconsin farmers, manufacturers, businesses and workers".
The original NAFTA deal was signed in 1992 by former President George H.W. Bush.
Trucks move almost $385 billion in goods between the US and Mexico, and $336 billion in trade between the USA and Canada.
If Trump ends NAFTA, Congress would have 60 days to approve USMCA or both trade deals would be void. While some argue a withdrawal must be approved by Congress, others point to a pattern of historical practice in which presidents have unilaterally withdrawn from treaties "for most of the 20century", said Bradley. MacNaughton said Canadian diplomats will rev up their machine to lobby members of Congress for their support.
In a blog entry posted shortly after the agreement was signed Friday, Cato Institute trade analyst Simon Lester appeared to anticipate Trump's move - although he acknowledged it would have made a lot more sense if the Republicans still had control of Congress.
Sen. Sherrod Brown, D-Ohio, has been critical of the new trilateral trade deal.
The signing of the trade agreement is largely ceremonial as it will need to be ratified by all three countries before it can formally take effect.
"Whatever they're calling it now, it has some kind of gobbledygook name - the trade agreement formerly known as Prince", Pelosi remarked Friday, "no, I mean, formerly known as NAFTA - is a work in progress". Mr Trump had told lawmakers he would veto any bill that did not include US$5 billion for a southern border wall, raising the prospect of a partial government shutdown.
"What isn't in it yet are enough enforcement reassurances regarding workers" and the environment, she told a press conference.
Another provision gives the United States the right to limit access for longhaul truck drivers from Mexico. Senator Marco Rubio of Florida tweeted on Friday that the deal as drafted would put Florida's seasonal vegetable growers "out of business".