Donald Trump: Can US president really declare national emergency to build US-Mexico border wall?

Donald Trump has again said he is considering declaring a national emergency in order to bypass Congress and begin construction on a wall across the US-Mexico border.

The US president has long promised to build a wall - and to have Mexico pay - but has so far failed to secure the funds following a standoff with Democrats in Congress, who consider it immoral and largely ineffectual in dealing with illegal immigration.

The row has led to a partial government shutdown, with Mr Trump refusing to sign off on a budget that would reopen a number of federal agencies, including the department that deals with border security, unless he gets $5.6bn (£4.4bn) in funding to pay for the wall.

After initially threatening the keep the government shut for “months or even years”, Mr Trump is now suggesting he could go over the head of Congress and construct the wall under extraordinary presidential powers intended only to be used in case of a national emergency.

But would such an unprecedented move be legal?

Though it is unclear what exactly Mr Trump has in mind, Bruce Ackerman, a professor of law at Yale, says any move to use the military to build a wall would be illegal and would put service members at risk of committing a federal crime.

In a column for the New York Times, Mr Ackerman claims US constitutional tradition has “profoundly opposed” the president using the military to enforce domestic law, except when authorised by the constitution itself or by Congress.

But Elizabeth Goitein, a co-director at the Brennan Center for Justice, a law and public policy institute, says a US president has “complete discretion” to declare a national emergency, despite such an announcement in relation to Mr Trump’s wall being an “abuse of emergency powers”.

In 1976, in light of Richard Nixon’s abuse of office, Congress passed the National Emergencies Act, which forced future presidents to specify which powers they intended to use in case of a national emergency, as well as giving Congress the opportunity every six months to end it.

Despite apparently introducing some checks on presidential power, Ms Goitein, in a column for The Atlantic, says the law has failed “by any objective measure”.

There are 30 states of emergency in effect in the US today, most have been renewed for years on end, and Congress has failed to ever vote on whether to end any of them.

“This legal regime for emergencies—ambiguous constitutional limits combined with a rich well of statutory emergency powers—would seem to provide the ingredients for a dangerous encroachment on American civil liberties,” she writes.

Mr Ackerman, however, believes Congress would likely act if Mr Trump invoked the 1976 emergencies act. A Democrat-held House would likely vote down the move immediately, then pass the resolution onto the Senate, which he believes would see a majority vote against the president, despite it being Republican-held.

One law that could appear to give Mr Trump the potential to invoke emergency powers for the wall is a little-known law called the Insurrection Act.

It allows the president to deploy troops inside the US to suppress any “unlawful combinations” or “conspiracies” that “obstructs or hinders the execution of the law”.

The statute, however, does not define the terms, making it vague enough to potentially use against undocumented migrants crossing into the US from Mexico.

Legal powers available to the president are “ripe for abuse” during a national emergency, Ms Goitein said, adding: “There’s no legal definition of emergency, no requirement that Congress ratify the decision, and no judicial review.”

Much like Mr Trump’s travel ban, a wall built using emergency powers would almost certainly face a litany of legal challenges, giving courts the opportunity push back against the president’s authority.

"The president's authority in this area is intended for wars and genuine national emergencies," Evan Hollander, communications director of the House Appropriations Committee, told CNN. "Asserting this authority to build a wasteful wall is legally dubious and would likely invite a court challenge."