Democratic Rep. Jan Schakowsky of IL, for one, said ahead of the signing that it does "not do almost enough to raise wages for workers, lower costs for healthcare consumers, or protect the environment", and that barring major changes, the deal "will result in more broken promises by Donald Trump to American workers".
The pact will lock in US market access to Canada and Mexico, expand American exports and includes new measures to ensure fair competition, explained Trump as he stood alongside Canadian Prime Minister Justin Trudeau and Mexican President Enrique Pena Nieto.
Now that the it has been signed, the agreement will head to Capitol Hill, where Republican lawmakers hope to get it approved before the new Congress takes over in January.
He also said, "the task isn't done, more hard work is ahead". Trump had commented how the US did not like Canada's negotiation style.
Canadian Prime Minister Justin Trudeau was less enthusiastic about the new regional trade pact on Friday, but said the USMCA would resolve the threat of "serious economic uncertainty" that "would have gotten more damaging".
This has been a battle, and battles sometimes make great friendships.
The United States-Mexico-Canada Agreement (USMCA) opens 3.6 per cent of Canada's dairy market to tariff-free imports and limits Canada's global dairy exports of skim-milk powder, baby formula and milk-protein concentrates.
Meanwhile, Mr Trump's other major trade-related meeting during the G20 summit will be with China's leader Xi Jinping on Saturday, with analysts looking for any sign that the United States president will hold off on plans to toughen USA tariffs on Chinese imports in January.
The three countries continued to talk out the agreement all the way up to the signing ceremony.
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Trump has made renegotiating NAFTA a centrepiece of his presidency.
"This has been a battle", Trump said in remarks right before the leaders put pen to paper.
Despite Trump's hostile rhetoric about unauthorized immigrants crossing into the United States from Mexico, he appeared to be on better terms with that nation's president, Enrique Peña Nieto, who was serving his final day in office - and, in Trump's view, going out on a high note by signing the trade agreement.
But in a statement, US Treasury Secretary Steven Mnuchin said the updated pact "marks a critical step in modernising and rebalancing North American trade".
The U.S. president, in bombastic style on Friday, called it "groundbreaking" and debatably "the largest, most significant and balanced trade agreement in history".
Mexico has also agreed to labor reforms, while Canada agreed to increase market access for US "dairy products, eggs, and poultry". The news ensured any meeting with Putin would have put a spotlight on the U.S. special counsel's investigation into whether the Trump campaign colluded with Moscow during the election.
"We've taken a lot of barbs and a little abuse and we got there", Trump said thanking Trudeau and Pena Nieto.
The president canceled on Putin not long after his former lawyer, Michael Cohen, revealed he had lied to Congress to cover up that he was negotiating a real estate deal in Moscow on Trump's behalf during the Republican presidential primary in 2016.
Mexican negotiators reportedly desire modifications to limit future metal shipments to the United States to 80 percent of current levels, while Canada is resisting quotas in the face of low-priced steel from China entering the American market.
She added that she will vote on the deal only if President Trump revises it.