In 1976, Congress enacted the National Emergencies Act, which authorizes the president to declare emergencies for a variety of purposes — great and small — but offers no definition of emergency.

Any court battle would probably be fought around that definition and its validity, according to Walter Dellinger, head of the Justice Department’s Office of Legal Counsel during the Clinton administration.

That office, which advises the president on constitutional issues, “has an obligation to determine that there is actually a basis” for a declaration of emergency “and to resign if there is not.” It would be “a critical moment for the rule of law” both for them and the attorney general, Dellinger said.

Bobby Chesney, a professor at the University of Texas School of Law, agreed that much would depend on how the Trump administration defines “emergency” and whether the claim is convincing to the courts.

“I think most of us fully understand there is no emergency,” he said. “But it doesn’t follow that everyone will see it that way.”