"Battles sometimes make great friendships", Trump said.
He called the deal by its old name NAFTA, prodded Trump over USA steel and aluminium tariffs, and said General Motors Co's decision to cut production and slash its North American workforce, including in Canada, was a "heavy blow". "Donald, it's all the more reason why we need to keep working to remove the tariffs on steel and aluminum between our countries".
At the event, on the sidelines of an worldwide summit opening in Buenos Aires, Trump touted the hard-fought deal as "a truly groundbreaking achievement" and proudly held up the signed agreement for the cameras at the conclusion of a short ceremony.
A day before Canada, the United States and Mexico are due to sign a new trade pact, negotiators are still thrashing out what exactly they will be putting their names to, officials said on Thursday.
Earlier this month, a coalition of no fewer than 40 Republicans in Congress wrote to Trump urging him not to sign the agreement, expressing disdain for "inappropriate" and "insulting" language that commits the three countries to supporting and protecting sexual orientation and gender-identity rights. Uncertainties remain, as the original 1994 pact remains in effect, and tariffs on steel and aluminum continue to be a major irritant.
Congress is likely to wait to vote on the deal until the United States International Trade Commission releases a report on the economic impact. "In the United States, the new trade pact will promote high-paying manufacturing jobs and promote greater access for American exports across a range of sectors including our farming, manufacturing and service industries".
'The new agreement will assure a new prosperity, ' he added.
"We've taken a lot of barbs and a little abuse and we got there", Trump said thanking Trudeau and Pena Nieto.
The president falsely claimed the new deal was "the largest trade deal ever made" (the Trans-Pacific Partnership, from which he withdrew the United States, was larger), and asserted that it will lead to "high wages and higher wages" in the auto industry and bring back jobs that have been moved overseas.
Even at the signing ceremony, there were echoes of the uncomfortable background to the USMCA, which Trump has made exhibit number one in his case to voters that he is putting "America first".
The deal - which replaces the North American Free Trade Agreement (NAFTA) - resulted from more than a year of renegotiation discussions.
"There is still a ways to go to gain support in the new Congress for this agreement", said Representative Bill Pascrell, the top Democrat on the House Ways and Means trade subcommittee.
Nieto explained the USMCA "is the first trade agreement that incorporates elements that address the social impact of global trade; it enables the participation of more sectors in the economy".
As the G20 summit began in Argentina on Friday, the USA president told reporters that the deal "changes the trade landscape forever".
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. In the USA, as Politico notes, that could mean "months of fierce debate between the Trump administration and Congress".
In a letter released Thursday, the Dairy Farmers of Canada warned that the text of the deal gives the US too much say over how the Canadian system is managed, and urged the government to stand down until that provision is excised. "That should be true for NAFTA and true for every deal we cut".