WASHINGTON (Reuters) - President Donald Trump will almost certainly face legal challenges over his decision to declare a national emergency to get additional funding for a U.S.-Mexico border wall, circumventing the power of Congress to set spending policy.

FILE PHOTO: Workers and U.S border patrol officers stand next to an excavator working in a section of the new wall between El Paso, Texas, in the United States and Ciudad Juarez as seen from the Mexican side of the border in Ciudad Juarez, Mexico, February 5, 2019. REUTERS/Jose Luis Gonzalez

Legal scholars say it is unclear how such a step would play out, but they agree 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.

Trump is unhappy with a bipartisan border security bill that is going through Congress to avert another government shutdown, because it contains only a fraction of the funds he demanded for his promised border wall. The White House said Trump would sign the bill but declare a national emergency to try to obtain funds for the wall.

That will likely trigger a long legal fight possibly stretching into Trump’s 2020 re-election bid, and embolden critics who already accuse him of authoritarian tendencies and unpredictable swerves in policy-making.

Congressional Democrats are already vowing legal challenges.

They have balked at giving Trump money for what they say is a wasteful and unnecessary wall.

Trump made his promise to build a wall and have Mexico pay for it a centerpiece of his 2016 presidential campaign. The Mexican government has refused to pay.

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.

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 are split.

Robert Chesney, a professor of national security law at the University of Texas, said a legal challenge on those grounds might succeed but 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.

While the U.S. Supreme Court has denied legal standing to individual lawmakers from the House of Representatives challenging White House actions, the entire House may have more solid legal ground to bring a lawsuit.

In 2015, the federal appeals court in Washington said the House as an institution had standing to challenge a means chosen by the Obama administration to fund part of the Affordable Care Act, where Congress had not appropriated the money.

Individuals or businesses with contracts canceled because of a redirection of military funds could also challenge Trump in court, as could private landowners whose property might be seized, Chesney said.

A practical issue for Trump, even if he could credibly argue 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.