Once considered a done deal, the replacement for the North American Free Trade Agreement has hit a roadblock after U.S. House Speaker Nancy Pelosi declared that there will be no vote in Congress unless Mexico changes its labour laws.

Pelosi hinted at an event in Washington, D.C. that the new trade deal may have to be reopened to satisfy Democrats, but Canada’s Ambassador to the U.S. David MacNaughton says not a chance.

"We have said a deal's a deal and we're not going to reopen, and because once you do that who knows where that goes," MacNaughton said to reporters on Capitol Hill late Tuesday.

Behind closed doors, MacNaughton says he is reminding American lawmakers that the clock is ticking.

NAFTA’s replacement, the United States-Mexico-Canada Agreement (USMCA), must be ratified in Canada before Parliament breaks in mid-June, ahead of the federal election.

If the deal is not ratified by June 14, MacNaughton says implementation of the agreement could be delayed for years.

"If there isn't some momentum created around this, it’s going to be hard to get it through," said MacNaughton.

A growing number of House Democrats are pushing back against the new trade deal.

"We weren't in office when many of the components of the new NAFTA were being negotiated," said Michigan Democrat Haley Stevens. "This is a chance for us to ask questions."

Democrats say they are worried about enforcement and the impact on drug prices, environmental protections of the yet-to-be-ratified revised free trade deal known as the USMCA.

"If Congress is going to support a new trade agreement it has to be enforceable, and it’s got to be fair to our workers," said Washington Democrat Ed Lieu.

New York Democrat Alexandria Ocasio-Cortez says the current trade deal does not have her vote.

"There are giveaways that will skyrocket the price of pharmaceuticals and a giveaway to big pharma," says Ocasio-Cortez.

Democrats don't seem to be in a rush to vote on the new trade deal -- or to give U.S. President Trump a win -- anytime soon.