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President Donald Trump claimed he has the “absolute right” to “order” US companies to stop doing business with China that would involve using his broad executive authority in a new and unprecedented way under a 1977 law.

On Friday, China unveiled a new round of retaliatory tariffs on about $75 billion worth of US goods, the latest escalation in an on-going trade war that’s putting a strain on the world’s two largest economies. In response, Trump wrote on Twitter later Friday: “Our great American companies are hereby ordered to immediately start looking for an alternative to China including bringing …your companies HOME and making your products in the USA.”

When leaving the White House for the G7 summit in France, Trump told reporters, “I have the absolute right to do that, but we’ll see how it goes.” He later explained that he was referring to the 1977 International Emergency Economic Powers Act (IEEPA), and in a Friday tweet wrote: “For all of the Fake News Reporters that don’t have a clue as to what the law is relative to Presidential powers, China, etc., try looking at the Emergency Economic Powers Act of 1977. Case closed!”

Trump’s latest comments again raise questions as to how far the President’s authority goes under the IEEPA. In May, Trump threatened to slap Mexico with punitive tariffs unless it slowed the passage of migrants from Central America to the US. The IEEPA, according to the Congressional Research Service, has never been invoked to impose tariffs, and Trump ultimately drew back at the last minute.

The IEEPA, passed in the wake of Watergate and Vietnam, gives Trump “broad authority to regulate a variety of economic transactions following a declaration of national emergency,” according to an analysis by the CRS.

Those presidential powers can be used “to deal with any unusual and extraordinary threat….to the national security, foreign policy, or economy of the United States.”

Under the IEEPA, the President has to consult with Congress before invoking his authority and, after declaring a national emergency, send a report to Congress explaining why.

This authority has been used frequently; there have been 54 national emergencies, 29 of which are ongoing. In the first use of the IEEPA, during the Iran hostage crisis in 1979, President Jimmy Carter imposed trade sanctions against Iran, freezing Iranian assets in the US, according to CRS.

Stephen Vladeck, a law professor at the University of Texas and a CNN legal analyst, told CNN in May that what Trump wanted to do under the law with Mexico may have been within the authority given to the White House by Congress – though it might not have been what Congress ever intended.

“The idea behind these authorities is that the President is better situated to make those kinds of determinations than Congress, especially when they’re time-sensitive,” Vladeck told CNN at the time. “So I think the President’s conduct may well be within the letter of the law here. But, as with the National Emergencies Act, I very much doubt this kind of exercise of the authority conferred by the statute is what Congress had in mind.”

On Saturday, Vladeck again weighed in, tweeting: “One of the enduring phenomena of the Trump era is going to be the list of statutes that give far too much power to the President, but that many didn’t used to worry about—assuming there’d be political safeguards. Today’s entrant: The International Emergency Economic Powers Act.”

Under the law, though, Congress can end an emergency with a joint resolution.

Former Massachusetts Gov. Bill Weld, who has mounted a longshot bid against Trump for the 2020 Republican presidential nomination, called it “outrageous” that a US President would tell US companies how to conduct business.

“That he believes he can actually carry out such an outrage is the insanity of a would-be dictator,” Weld tweeted Saturday.

CNN’s Zachary B. Wolf and Nikki Carvajal contributed to this report.