TODAY’S EXPLAINER,

The question has arisen about whether President Donald Trump has emergency powers to build a proposed wall on the Mexican border without lawmakers’ approval.

What happens when a president declares a national emergency? What are the rules that govern the move?

What are the president’s emergency powers?

The president has the authority to declare a national emergency, which activates enhancements to his executive powers by essentially creating exceptions to rules that normally constrain him. The idea is to enable the government to respond quickly to a crisis.

The National Emergencies Act, enacted during the post-Watergate reform era, regulates how presidents may invoke such powers. It requires them to formally declare a national emergency and tell Congress which statutes are being activated.

How could the administration make its case?

The Trump administration could point to two laws, both involving the military, and say they allow officials to proceed with building a border wall without first obtaining explicit authorization and appropriations from Congress, according to Elizabeth Goitein, who is a co-director of the Brennan Center’s Liberty and National Security Program.

One of the laws permits the secretary of the Army to halt Army civil works projects during a presidentially declared emergency and instead direct troops and other resources to help construct “authorized civil works, military construction and civil defense projects that are essential to the national defense.”

Another law permits the secretary of defense, in an emergency, to begin military construction projects “not otherwise authorized by law that are necessary to support such use of the armed forces,” using funds that Congress had appropriated for military construction purposes that have not yet been earmarked for specific projects.

In light of those statutes and similar ones that give presidents flexibility to redirect funds in a crisis, the Trump administration could point to serious arguments to back up the president. Is the president’s legal authority clear?

No. If he invokes emergency powers to build a wall, Trump is almost certain to invite a court battle. While Goitein agreed that “there is a nonfrivolous legal case to be made” that emergency-powers laws might empower Trump to spend military funds on a wall, she also pointed to counterarguments.

For example, under one of the laws Trump might try to invoke, the military may redirect funds to build only projects that Congress has separately authorized. Lawmakers have not approved a military wall spanning the border.

Who decides what constitutes an emergency?

If Trump declares that the situation along the border constitutes an emergency that justifies building a wall without explicit congressional sanction, he will run up against a reality: that the facts on the ground have not drastically shifted. The number of people crossing the border unlawfully is far down from its peak of nearly two decades ago.

But before a court could decide that Trump had declared an emergency under false pretenses, the court would first have to decide that the law permits judges to substitute their own thinking for the president’s in such a matter. The Justice Department would surely argue that courts should instead defer to the president’s determination.