“IEEPA was not designed for the imposition of tariffs,” said Jennifer Hillman, a Georgetown University law professor. “It was intended to give the President the power to impose economic sanctions when the President finds an ‘unusual and extraordinary threat’ to the national security, foreign policy or economy of the U.S. from foreign sources,” she said in an email.

“There is no reference” in the law to “presidential authority to impose tariffs,” said Edward Alden, a senior fellow at the Council on Foreign Relations. “It has been used for economic sanctions of various sorts against enemy countries, Iran, Iraq and Sudan and Libya. It’s never been used against a close ally.

“This is a considerable step beyond anything the president has done before in terms of abusing the authority delegated to him by the Congress. I think that’s why you’re seeing a reasonably significant reaction on the Hill.”