Facing stiff resistance from Congress, President Donald Trump has said he might use his emergency powers to construct hundreds of miles of wall on the southern border to prevent migrants from crossing into the United States illegally from Mexico.
Trump could indeed declare a national emergency, citing what the administration calls a "crisis" at the border, after nearly 103,000 people were detained in October and November after entering the country illegally.
But trying to build a wall in this way would face significant legal barriers.
Trump does have emergency powers
The National Emergencies Act allows the president to declare a national emergency, providing a specific reason for it.
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That then allows the mobilisation of hundreds of dormant emergency powers under other laws. Those can permit the White House to declare martial law, suspend civil liberties, expand the military, seize property, and restrict trade, communications and financial transactions.
But the powers are not unlimited, and can be blocked by Congress and the courts. During the Korean War in 1952, President Harry Truman sought to take over US steel factories to keep them producing in the face of a planned national strike by industry workers.
Steel companies took the case to the Supreme Court, which ruled in their favour, saying the president's emergency powers did not allow him to seize privately-owned plants to avert a strike.
All presidents use the National Emergencies Act
Every recent president has used the NEA, and more than two dozen states of emergency are currently active, renewed annually.
President George W. Bush invoked it after the September 11, 2001 Al-Qaeda attacks to be able to expand and ready the military beyond what was budgeted, and to undertake secret surveillance and employ interrogation methods on detainees widely denounced as torture.
President Barack Obama tapped the NEA to declare an emergency in 2009 over the swine flu threat, giving authorities and hospitals extra powers to act quickly against the outbreak.
Most often, the NEA has been used in actions against other countries.
One NEA emergency in place since 1979 has restricted trade with Iran. Another, dating to 2006, blocks property of people who were deemed to be undermining democracy in Belarus.
Powers, funds limited
If Trump declares a national emergency, he could deploy more manpower to the border. But to build a wall, he would still have to find billions of dollars to fund it.
One emergency law permits the president to order "military construction projects" using funds already available in the military budget.
But could the wall be called a "military" project? There are strong restrictions on the US military and its funding being deployed for domestic, non-defense purposes, though emergency laws sometimes permit it.
Moreover, building the wall will require taking control of privately owned land that abuts much of the border, which could force years-long legal battles with landowners.
Challenge from Congress
The NEA gives Congress the right to immediately challenge a presidential emergency declaration.
Given that Congress has already declined to fund the wall, a challenge would likely quickly pass the Democratic House. Then the Republican-controlled Senate would have to decide whether they agree with the president's invocation of emergency powers to build a wall they haven't given him money for.
"That's a non-starter," senior Democratic Congressman Adam Schiff said Sunday.
"If Harry Truman couldn't nationalize the steel industry during wartime, this president doesn't have the power to declare an emergency and build a multibillion-dollar wall on the border."
