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 Mr. Trump if he invokes emergency powers to build a wall, said William C. Banks, a Syracuse University law professor who helped write a 1994 book about tensions between the executive and legislative branches over security and spending, “National Security Law and the Power of the Purse.”
“The fundamental principle is that no president or official may spend funds that were not appropriated for that purpose,” he said. “But I think that it’s possible that the president could declare a national emergency and then rely on authority Congress has historically granted for exigencies to free up some funds to support constructing a barrier along the border.”
Is Trump’s legal authority clear?
No. If he invokes emergency powers to build a border wall, Mr. Trump is almost certain to invite a court battle. While Ms. Goitein agreed that “there is a nonfrivolous legal case to be made” that emergency-powers laws may empower Mr. Trump to spend military funds on a wall, she also pointed to counterarguments any lawsuit would have to contend with.
For example, she noted, under one of the laws Mr. 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.
Still, the administration might argue that Congress has effectively preapproved a wall-like barrier under other laws, including one that authorizes the military to construct border “fences” blocking drug-smuggling corridors, and another, the Secure Fence Act of 2006, that empowers the Department of Homeland Security to build “physical infrastructure enhancements” along the border.
The government could skip the requirement to identify pre-existing authorization for a wall if it invoked a different emergency-powers law for the funds, but that route would raise other problems, Ms. Goitein said. Among them, the government would need to show that a wall meets the legal definition of military construction even though it is not clearly tied to a military facility or installation, and that the southern border situation represents the kind of emergency that requires the use of the armed forces.
Does it matter if there is no true emergency?
If Mr. Trump declares that the situation along the southern border suddenly 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. The recent caravans from Central America primarily consist of migrants who are not trying to sneak across the border, but instead are presenting themselves to border officials and requesting asylum.