US President Donald Trump has floated the possibility of declaring a 'national emergency' to get his wall on the border with Mexico built.
He hasn't yet, saying he'd like to get the funding for it passed through Congress, but that's looking increasingly unlikely.
"We're looking at a national emergency because we have a national emergency - just read the paper," Mr Trump said at the weekend, citing an "invasion" at the southern border.
Illegal immigration into the US has actually been in decline for more than a decade, but that hasn't stopped Mr Trump from pushing for a wall, which could cost as much as US$25 billion according to some estimates.
How declaring a national emergency could help get the wall built
Under the US system of government, the President heads up the executive branch. In theory he wields one-third of the power, the other two-thirds equally divided between the judicial (courts) and legislative (the Senate and House) branches.
But under the 1976 National Emergencies Act the President has the right to declare a national emergency, unlocking up to 136 extra statutory powers, according to New York University's Brennan Center for Justice.
There are two in particular legal experts say Mr Trump would likely be considering.
One allows the President to redirect money set aside for military construction projects, the Washington Post reports, with the requirement the armed forces do the actual building. The US$5.6 billion Mr Trump wants would use up more than half of the military's construction budget.
A second statute gives him the ability to take money intended for US Army civil works projects and reassign it to projects "essential to the national defence". But according to Bloomberg, the amount Mr Trump wants would use up all the funds allocated this year, much of it already earmarked for ongoing projects - and he'd need Congressional approval to change that.
What else Trump could do
Building the wall might just be one extreme measure Mr Trump takes to stop the so-called "invasion". The Atlantic's round-up of powers available to the US President under a national emergency makes for sobering reading.
Technically, the options include:
- shutting down critical websites, or requiring search engines to always return pro-Trump results
- send the military into 'sanctuary cities' that don't crack down on illegal migrants, or to break up anti-Trump protests
- suspending the usual court systems in favour of martial law
- lifting the prohibition on testing biological and chemical weapons, including on human subjects
- blocking financial transactions and freezing assets.
Will it work?
There is debate over whether Mr Trump could legally invoke many of the statutes over illegal immigration. Writing for the New York Times, Yale law professor Bruce Ackerman said any soldiers that helped build the wall under a national emergency would be committing a federal crime and face up to two years in jail.
"If President Trump declared an emergency, Section Five of the act gives the House of Representatives the right to repudiate it immediately, then pass their resolution to the Senate - which is explicitly required to conduct a floor vote within 15 days.
"Since President Trump's 'emergency' declaration would be a direct response to his failure to convince Congress that national security requires his wall, it is hard to believe that a majority of the Senate, if forced to vote, would accept his show of contempt for their authority."
Writing for NBC News, legal analyst Danny Cevallos said any national emergency would be declared against the will of Congress, and quickly be shut down by the Supreme Court.
"The Supreme Court would have to deny any congressional power to act in the controversy to sustain the President's exclusive control. In this category, when the President acts in defiance of Congress, he should usually lose."
Not that rare
Though Mr Trump's threat of a national emergency has made headlines, it's not actually a particularly rare route for US Presidents to take. Barack Obama declared 12 states of emergency, and his predecessor George W Bush 13.
There are currently 31 active national emergencies in the US, with one of them dating back to Jimmy Carter and relating to the Iranian Revolution.
Bill Clinton declared 17, the oldest still in effect relating to the proliferation of weapons of mass destruction.
Mr Trump has declared three national emergencies so far - sanctions on 13 generals and world leaders accused of mass executions and murder, sanctions on people involved in social media campaigns intended to influence elections, and "blocking property of certain persons contributing to the situation in Nicaragua".
He did not declare a national emergency over the growing number of people using opioid drugs, leaving it at the level of a public health emergency.
Previous use of national emergencies
1933: Franklin Roosevelt declares an emergency so he can shut banks, stopping the Depression from getting even worse.
1957: Dwight Eisenhower sends troops to Little Rock, Arkansas, to ensure black children can go to school.
1970: Richard Nixon calls in the National Guard to deliver mail, breaking a postal strike.
1992: George Bush deploys troops to stop the Rodney King riots.
2001: George W Bush gives himself expanded powers following the 9/11 attacks.
2009: Barack Obama declares an emergency to help stop the swine flu epidemic.