President Donald Trump declared a national emergency on Friday to redistribute government funds to build his long-sought after wall along the U.S.-Mexico border, but experts question whether it’s constitutional.

“So we’re going to be signing today, and registering, national emergency and it’s a great thing to do, because we have an invasion of drugs, invasion of gangs, invasion of people and it’s unacceptable,” Trump said in a Rose Garden address.

In a statement, Rep. Tom O’Halleran, D-Flagstaff, said this action would divert resources from the Department of Defense and law enforcement programs to fund a border wall.The statement also said that $3.5 billion from the Pentagon’s military construction fund, $2.5 billion from the Pentagon’s drug interdiction initiative and $600 million from the Department of the Treasury’s drug forfeiture program will be shuffled to fund the construction of the border wall.

Although the National Emergencies Act of 1976 grants the president broad authority to declare a state of national emergency, there is debate as to whether any federal laws grant the president the authority to redirect funds to the border wall construction and to instruct the military to build the wall.

The Brennan Center for Justice at the New York University Law School, a law and public policy center that specializes in constitutional issues, compiled a list of federal laws and the powers they grant to presidents that can be used in a time of war and during national emergencies.

The exact statutory powers that President Trump plans to use aren’t yet known because the emergency text has not yet been made public.

That uncertainty worries Jon D. Michaels, professor at the UCLA School of Law, because he said the president has not shown the same level of responsibility, discretion and caution as previous presidents who have declared a national emergency.

“The level of responsibility that has been exercised (by earlier presidents) has been sufficient that we haven’t worried about all these extra statutory authorities kicking in because they’re not being exploited,” Michaels said.

The Brennan Center said that statutory powers 10 U.S.C. 2808 (a) and 33 U.S.C. 2293 might offer some legal cover for the president’s wall-building ambitions.

During a state of emergency, statutory power 10 U.S.C. 2808 (a) says that the secretary of Defense may undertake military construction projects that are necessary to support such use of the armed forces.

And statutory power 33 U.S.C. 2293 allows the Army to terminate or defer any Army civil works project and apply the resources to authorized military construction, civil defense projects that are essential to national defense.

Some constitutional law experts say there is no explicit power granted to the president by any federal laws or by the constitution, to use the military to build the border wall.

Professor Bruce Ackerman, Sterling professor of law and political science at Yale University and a constitutional expert, said Trump’s use of a national emergency presents a grave constitutional problem.

“It is illegal for the commander in chief to use military resources for domestic law enforcement, except where explicitly authorized,” he said, adding that no explicit authorization exists.

Ackerman said this will provoke a constitutional crisis involving the military if Trump orders his generals to begin construction on new border barriers.

“The important point is that the generals are going to have to decide immediately: Are they going to follow the law of the United States or are they going to follow the commander in chief?”

Because Trump “only pointed to issues of domestic law enforcement, criminal enforcement and the like in his remarks today,” Ackerman said, it would be an unconstitutional use of his executive powers.

In 1952, he said, the Supreme Court ruled in Youngstown Sheet & Tube Co. v. Sawyer that a president can’t use his commander in chief powers for domestic law enforcement. The case arose during the Korean War, when President Truman ordered the secretary of Commerce to seize and operate most of the nation’s steel mills to avert a expected steelworker strike.

Citizens for Responsibility and Ethics in Washington, a government ethics watchdog based in Washington D.C., already has filed suit against the Department of Justice for failing to provide documents outlining the legal authority of Trump’s emergency declaration. The American Civil Liberties Union and Gov. Gavin Newsom, D-Calif., announced they intend to file separate lawsuits in the coming weeks.

Once a challenge is filed in court, Ackerman said, “the Supreme Court is going to decide in six months to a year.”

In the meantime, the national emergency declaration can be undone if both the Senate and House of Representatives pass a joint resolution and Trump signs it. If he vetoes the joint resolution, it would require two-thirds of both the House and Senate to override the veto.

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