President Donald Trump, Canada’s Prime Minister Justin Trudeau, right, and Mexico’s President Enrique Pena Nieto, left, participate in the USMCA signing ceremony last week in Buenos Aires, Argentina.

President Donald Trump’s threat to terminate the existing North American Free Trade Agreement puts pressure on U.S. lawmakers to limit the changes they want in a new regional pact signed to great fanfare last week by the U.S., Canada and Mexico.

Trump declared late Saturday that he’d soon notify Congress of his intention to terminate NAFTA, a move that would give lawmakers six months to bless the deal to replace it. Even though the new agreement was signed by Trump and other leaders on Nov. 30, it must also be ratified by lawmakers in the three countries. Trump’s essentially leaving them a choice: take the new deal or no deal at all.

In the U.S., the new agreement is all but certain to be taken up by Congress next year, when Democrats regain a majority in the House. Key Republicans and Democrats are withholding their support from the new pact — known as the USMCA — and want to extract changes, most likely through legislation needed to implement the deal.

Democratic Sen. Sherrod Brown of Ohio, a NAFTA critic who has said he worked with the administration in renegotiating the deal, said Sunday that “the work’s not done yet” and there’s still an opportunity to ask Mexico to strengthen labor requirements.

The USMCA “doesn’t live up to the promise the president said of a renegotiated NAFTA,” Brown told CNN’s “State of the Union” without saying how he plans to vote.

The path for Trump to kill the current NAFTA is muddied. Under the existing agreement, the president can give six months notice of withdrawal, although that’s not binding — he can give it and then never actually withdraw.

If he did quit, U.S. lawmakers would have to repeal laws that enact it and might balk at doing so. It raises the prospect of Trump quitting, nullifying parts of the pact while other elements linger under U.S. law amid a fight with Congress — or of him giving notice of quitting and then, after six months, declining to do so.

Trump’s termination threat, if carried out, would essentially remove a safety net from under the new agreement’s journey through Congress, leaving lawmakers less leeway to demand revisions. U.S. Trade Representative Robert Lighthizer, an architect of the deal, is open to changes, but only to a point. The U.S. can tuck some changes into the trade deal’s implementing legislation and request that Mexico and Canada go along with it.

“The negotiations are not going to be reopened, right? The agreement’s been signed,” Lighthizer said Friday after the new accord was signed at the Group of 20 summit in Buenos Aires.

“We’ll get the support of a lot of Democrats, a very high number of Democrats. Absolutely, just no doubt about it.”

Named by Trump as the U.S.-Mexico-Canada Agreement — though neither Canada nor Mexico is calling it that — the deal will replace the 1994 NAFTA pact in the three countries, which trade $1 trillion annually.

The new deal got early backing from one key figure in the debate: Republican Sen. Chuck Grassley of Iowa, who beginning in January will chair the Senate Finance Committee that will consider the pact. Grassley vowed to usher the deal through Congress, saying it’ll benefit the U.S. economy and bolster agriculture interests in the Midwest.

Lighthizer didn’t say what he might change to win Democrats’ support, nor did he indicate a timeline for how quickly the ratification would proceed. He said he’s already in talks with Democratic leaders, though.

Lawmakers in both parties are still reviewing the details, and many are making no promises as they work to ensure exporters in their home states can thrive under the proposed trade ground rules.

“There is still more work to be done in Congress to ensure any final agreement stops the outsourcing of jobs to other countries, strengthens Buy America, puts in place real enforcement of labor provisions, and allows the United States to take action on currency manipulation,” Sen. Tammy Baldwin, D-Wis., said in a statement.

“I will only support this new deal if we can make it a better deal for Wisconsin farmers, manufacturers, businesses and workers.”

Two other Democratic senators, Mark Warner of Virginia and Sherrod Brown of Ohio, fell short of a commitment to back the agreement on Sunday. Warner said on CBS that he’s “not ready at all” to say how he’d vote, while Brown said on CNN that the agreement “doesn’t live up to the promise” made by Trump. “The work’s not done yet,” Brown said.

The deal signed Friday included some tweaks from the one struck two months earlier.