Critics of President Trump’s U.S.-Mexico-Canada Agreement are pushing to re-write the deal when it comes before Congress, despite the fact that existing trade law supposedly limits lawmakers to a simple up or down approval vote. Lawmakers and their allies say there is still wiggle room to add things in the deal's fine print.

Lawmakers hope to alter the deal through the one part of it that hasn’t been finished: the section on how its rules will be enforced.

Sen. Pat Toomey, R-Pa., said Wednesday in a Wall Street Journal op-ed he would support USMCA if the president adds "pro-trade modifications in the implementing legislation."

Democrats, however, want to use implementing language to push in the other direction. Rep. Bill Pascrell, D-N.J., who will likely take over the Ways and Means Committee next year, said last week he’ll push for strong implementation language in USMCA to force Mexico to raise worker wages.

Testifying before the International Trade Commission, Pascrell called for the inclusion of "strong, enforceable labor provisions that can be implemented and monitored meaningfully. That includes reforms to the system of suppressing wages and blocking labor organizing in Mexico."

Trade Promotion Authority, the law that covers submitting deals to Congress, generally gives the White House a strong hand in getting deals approved, hence its nickname, “fast track." But contrary to many reports, it doesn’t cut Congress out of the negotiation process entirely, experts say.

The implementing bill cannot change any legal terms of the deal, but can be used to force the administration to go back and further negotiate a deal in order to get it in line with the implementing language, said Lori Wallach, director of Public Citizen’s Global Trade Watch. Wallach pointed out that exactly that happened several times under Presidents George W. Bush and Barack Obama.

“For example, with the [South Korea trade] agreement, the Obama White House had to contact Korea and say, ‘Hey, here’s the deal. I know you already renegotiated this once, but we’re going to have to do it again,” Wallach said. “So they basically re-opened the agreement.”

Toomey argued Friday that the president should force a vote on the deal in the upcoming congressional lame duck session to avoid this scenario. "[K]ey House Democrats are demanding changes that would undermine trade among the three countries — and may even require reopening negotiations," he warned.

The implementing language itself can also be used to subtly redefine the underlying terms of the deal. TPA’s implementing language is, according to the law, supposed to merely lay out how to enforce the deal’s terms. But how the enforcement is written is significant, said Bryan Riley, trade policy expert for the National Taxpayer’s Union.

“When Congress passed TPA it was understood that the implementing legislation for trade bills typically has to include some changes to legislation, for example changes in tariff rates,” Riley said. The language isn’t supposed to include unrelated or extraneous items, he said.

That still leaves wiggle room, said a trade policy lawyer who requested anonymity in order to comment on ongoing affairs. “The drafter of the legislation gets the benefit of deciding what is ‘necessary and appropriate.' What one considers ‘necessary and appropriate' is obviously always going to be a matter for debate,” the lawyer said. “If one wants to push the envelope, that’s how you would do it.”

Pushing that envelope is precisely what some groups like organized labor have been calling for. "There is a thing called the implementing language. That is a bill ... that can give strong implementing language, weak implementing language, or no implementation at all," Richard Trumka, president of the AFL-CIO, told the Washington Examiner earlier this month. "When we see the implementing language, then we will know if it is a deal we can and will support — or we don't."