Trump said he plans to terminate NAFTA, setting off a firestorm among lawmakers.

But both key Republicans and Democrats agree that he has no such legal authority.

Lawmakers also warned of the "disruptive" economic effect that a sudden NAFTA withdrawal could have.

WASHINGTON — Key Republicans and Democrats are in agreement that President Donald Trump does not have any legal authority to withdraw the United States from the North American Free Trade Agreement (NAFTA) in an effort to force Congress to swiftly consider his new deal with Canada and Mexico.

Members of the Senate Finance Committee told INSIDER that Trump cannot just shred NAFTA, but would instead have to consult with Congress, where he would no doubt face significant hurdles without a concrete and agreed upon replacement already in place.

Read more: Republicans are freaking out about Trump's massive new tariffs

"I'll be terminating it within a relatively short period of time. We get rid of NAFTA. It's been a disaster for the United States. It's caused us tremendous amounts of unemployment and loss and company loss and everything else," Trump told reporters this week. "That'll be terminated. And so Congress will have a choice of the USMCA or pre-NAFTA, which worked very well."

Republican Sen. Pat Toomey, who sits on the Senate Finance Committee, told reporters that Trump has no legal authority to withdraw, and he would require consent from Congress.

"The simple fact is NAFTA was enacted through legislation. It requires legislation to repeal it," Toomey said. "The bizarre thing about the administration’s contention is they acknowledge that if they changed one word in NAFTA they have to come back to Congress to get it approved, but somehow they can strike the whole thing and they don’t need Congress for that? That makes no sense and it is not consistent with the legislation."

And Toomey's feelings are felt across the political aisle. Oregon Sen. Ron Wyden, the top Democrat on the Senate Finance Committee, told INSIDER the Constitution's separation of powers is clear as day that Congress is in charge of NAFTA.

"Up at the White House, they ought to take out a copy of the Constitution. Article 1, Section 8, is very clear that Congress has the power to regulate foreign commerce and there’s also a Congressional Research Service analysis that supports it," he said.

The CRS analysis Wyden mentioned has stated any withdrawal would require congressional approval.

Wyden also said Trump is exhibiting lack of confidence in his United States–Mexico–Canada Agreement (USMCA) proposal.

"If the president was really confident that his trade proposal was going to work, he wouldn’t be playing this kind of brinkmanship and trying to throw this kind of muscle around," he said.

And Toomey suggested the president's rhetoric about wanting to tear up NAFTA is an attempt to kick-start consideration of the USMCA, but that it is a poor strategy to begin with.

"It appears the president is trying to say, 'you know it’s this or nothing.'" he said. "But when he doesn’t have the legal authority to enforce that choice, it's not a very good strategy."

But some suggested there is room for some kind of NAFTA dismantling by Trump, as long as he meets certain criteria.

"The president is given substantial authority in all of our trade dealings. But it’s not without restrictions," Massachusetts Sen. Elizabeth Warren told INSIDER. "In other words, certain factual findings are necessary first."

"As is often in the case, what the president proposes to do has to fit within a larger framework of what is legally permitted under the treaty," she added.

A rapid withdrawal from NAFTA could have devastating effects on the economy

In terms of the risks of a NAFTA withdrawal, Warren said the US needs a new deal to replace it, but without the risks of igniting a catastrophe that could devastate the economy.

"Look, we need a different NAFTA, but that means sitting down and negotiating for a NAFTA that works for American workers, small businesses, and farmers," she said. "Not just unilaterally just trying to start yet another trade war."

Toomey took a harder stance, saying a sudden or even timetabled withdrawal without a replacement could force the US economy into a downward spiral.

"It would be extremely disruptive," Toomey told INSIDER. "The markets would go haywire, I mean supply chains would be very, very disrupted. It’d be very harmful to the economy, to jobs in the United States."

Toomey has reiterated his sentiments to White House officials, but not yet to Trump.

"So it’s not a good policy," he added. "But as I say, I simply reject the idea he has the authority to do it."