The leaders of Mexico, Canada and the United States were due to sign a North American trade pact on Friday, although brinkmanship over the final details of the deal continued through the eve of the signing.
A Mexican source said that "the legal review, translations and collation of documents are being concluded", but ruled out important changes.
While Canadian Foreign Affairs Minister Chrystia Freeland will be at the event, the three leaders will sign the deal.
President Trump, his Mexican counterpart Enrique Pena Nieto and Canadian Prime Minister Justin Trudeau have officially signed the replacement to NAFTA, in the wake of the Trump administration enacting tariffs on steel and aluminum products from Canada and Mexico, which sparked retaliatory tariffs and negotiations with the two countries.
Friday's signing ceremony is a crowning moment for Trump, who loves to tout his deal-making prowess.
Canada's politically influential dairy lobby on Wednesday said the USMCA would "grant the USA oversight into the administration of the Canadian dairy system" and urged the government not to sign.
US tariffs on steel and aluminum remain in place, but a Canadian official said an advantage to signing onto the agreement now is an auto side letter exempting Canada of potential tariffs on exports of up to 2.6 million vehicles - well above current levels.
And dairy remains a sticking point with the influential Canadian dairy lobby urging Trudeau's government not to sign the trade deal as it gives the United States expanded access to Canada's dairy, egg, and poultry markets. Mexico swiftly retaliated with import tariffs on some U.S. goods, including steel, apples and pork.More news: Camera InSight sent a selfie from Mars
'A vast number of technical details need to be scrubbed and wrapped up, ' Freeland said. "The fact that this is an agreement in three languages adds to the level of technical complexity and it is on that level that we are just being sure that all the Is are dotted and all the Ts are crossed".
With a new deal on the table that's supposed to represent a new era of tariff-free trade, all three countries have been working on resolving the standoff before Friday.
Outgoing Mexican President Enrique Pena Nieto is attending the G20 meeting.
Renegotiating Nafta was a key pledge of Mr Trump's 2016 White House campaign.
'President Trump spoke with Canadian Prime Minister Trudeau this morning, ' spokesman Hogan Gidley said in a statement.
Mr Trudeau added that "the benefits of free trade must be broadly and fairly shared", before criticising United States tariffs on steel and aluminium.
The deal is a reworking of the 1994 North American Free Trade Agreement, which underpins $1.2 trillion in trade between the three countries.
He described General Motors' plant closures that had hit thousands of U.S. and Canadian families as a "heavy blow".