WASHINGTON (Reuters) – President Donald Trump would almost certainly face a legal challenge if he makes good on his threat to get funding for a U.S.-Mexico border wall by declaring a national emergency and circumventing Congress’s purse-strings power.

Legal scholars said it was unclear how such a step would play out, but they agreed a court test would likely focus on whether an emergency actually exists on the southern border and on the limits of presidential power over taxpayer funds.

Declaring an emergency would likely put an end to an 18-day partial government shutdown affecting 800,000 federal workers.

It could also result in a long court fight, one possibly stretching into Trump’s 2020 re-election bid and emboldening critics who already accuse him of authoritarian tendencies and unpredictable swerves in policy-making.

Trump triggered the partial shutdown last month by demanding that more than $5 billion in funding to pay for part of his wall be part of any legislation to fully reopen agencies whose funding expired for unrelated reasons on Dec. 22.

When Trump started vowing to build a wall, a promise his supporters cheered on the campaign trail, he pledged Mexico would pay for it. Mexico refused. Now Trump wants U.S. taxpayers to pay for the roughly $23 billion project.

PRESIDENTIAL DISCRETION

Under the Constitution, decisions about spending taxpayer funds and creating policy are typically made by Congress. But a 1976 law allows the president to bypass Congress and redirect funds in the event of a national emergency. The National Emergencies Act does not define “emergency,” giving the president broad discretion to declare one, legal experts said.

The law empowers Congress to override an emergency declaration, but that requires action by both chambers, which would be hard to get since the Senate is run by Trump’s fellow Republicans and the House of Representatives by Democrats.

The United States currently has about 30 national emergency proclamations in effect, including ones related to the Iran hostage crisis of 1979 and the swine flu pandemic in 2009.

Congress has made a wide range of special powers available to a president who declares a national emergency.

“I would guess that the White House legal counsel’s office is working overtime to go through each of these 400+ laws, looking for something that can be used for (the wall),” said Princeton University international affairs professor Kim Lane Scheppele.

One law allows the president to redirect U.S. Department of Defense construction funds that have not yet been allocated.

Another enables the U.S. Army to halt civil projects and instead apply the funds and personnel to projects “essential to the national defense.”

LITTLE PRECEDENT

There are few court cases on the scope of the president’s emergency powers and legal experts were split.

Robert Chesney, a professor of national security law at the University of Texas, said a legal challenge on those grounds might succeed, but added that the courts typically showed deference to the president on national security matters.

Elizabeth Goitein, a lawyer at the Brennan Center for Justice, said there were strong arguments that border wall construction is impermissible under various statutes granting the president emergency powers. “There are some very good arguments that those laws are not applicable here,” she said.

House Democrats have vowed legal challenges, but the U.S. Supreme Court in past cases has limited the ability of members of Congress to sue the president, said Chesney.

Individuals or businesses with contracts canceled because of a redirection of military funds might be better placed to challenge the president in court, as would any private landowners whose property might be seized, Chesney said.

FINDING THE MONEY

A practical issue for Trump, even if he could credibly argue that an emergency exists, is that he would need to get his wall money out of whatever funds are left over from a pool of about $10.4 billion in military construction projects during the current fiscal year, which ends on Sept. 30.

The U.S. military has not disclosed how much funding might be left over in its military construction budget. It was unclear whether any cash still available would be enough to make significant headway in building the border wall.