The National Emergencies Act specifies that a President can can declare a national emergency to engage in certain activities that are authorized under other statutes.

Stefanie Lindquist, foundation professor of law and political science at Arizona State University, believes the President would be invoking powers in association with U.S. law requiring the use of the Armed Services.

“So the way the national emergency could lead to the building of the wall is that, the argument would be made by the President that the wall is necessary as a national construction project because it’s necessary to support the use of the Armed forces,” she said.

Lindquist says there are currently about 30 outstanding executive orders that declare a state of emergency covering a variety of different circumstances.

"In terms of the specific provision about national construction projects, there have been two situations in which the President has declared a state of national emergency and used Department of Defense funds for purposes that were not otherwise specified in law," she said. "Those two incidences involved the Iraqi invasion of Kuwait and the other was after 9-11."

She said the Supreme Court has weighed in on previous emergency declarations.

"The most prominent example is what's known as the Youngstown Steel seizure case, in which President Truman, during the Korean War, issued an executive order to seize steel mills in the face of a strike. His argument was that it was necessary for them to maintain operations or else we would not have the military hardware needed for our troops in Korea.”

Lindquist said the Supreme Court struck that move down. She believes challenges to President Trump’s order could come from landowners who have property seized by eminent domain.