The U.S.-Mexico-Canada Agreement is meant to replace the 24-year-old North American Free Trade Agreement, which Trump has long denigrated as a "disaster."

Trump appeared with Canadian Prime Minister Justin Trudeau and outgoing Mexican President Enrique Pena Nieto at the Group of 20 nations summit in Buenos Aires for the formal signing ceremony. Each country's legislature must also approve the agreement.

"It's been long and hard. We've taken a lot of barbs and a little abuse and we got there," Trump said of the pact.

Legislative approval is the next step in the process, but could prove to be a difficult task in the United States, especially now that Democrats — instead of Trump's Republicans — will control the House of Representatives come January. Democrats and their allies in the labor movement are already demanding changes to the agreement.