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Washington (CNN)PresidentDonald Trumpclaimed 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, Chinaunveiled a new round of retaliatory tariffson about $75 billion worth of US goods, the latest escalation in an on-going trade war thats putting a strain on the worlds 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 well see how it goes. He later explained that he was referring to the 1977 International Emergency Economic Powers Act (IEEPA), and in aFriday tweetwrote: For all of the Fake News Reporters that dont 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!

Where Trumps emergency power on foreign business comes from

Trumps latest comments again raise questions as tohow far the Presidents 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 ananalysis 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 theyre time-sensitive, Vladeck told CNN at the time. So I think the Presidents 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 didnt used to worry aboutassuming thered be political safeguards. Todays 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, WeldtweetedSaturday.

CNNs Zachary B. Wolf and Nikki Carvajal contributed to this report.