However, that preceded the 1976 Act, which leaves the existence of an emergency wide open to interpretation.
“If President Trump declares a national emergency at the border, it is far from clear that courts would strike it down,” writes Chris Edelson, Assistant Professor of Government at American University School of Public Affairs.
Where would the money come from?
Shifting money from accounts to deliver the $5.7 billion he wants will be tricky.
The administration has been eyeing several pots of money – including disaster funds, counter-narcotic accounts and military construction dollars – to fund Mr Trump’s wall, according to congressional aides and White House officials.
One possibility is shifting a portion of the $13 billion in disaster aid Congress approved last year for Puerto Rico and a dozen states, including California and Texas, hit hard by hurricanes, flooding and other disasters. The money funds Army Corps projects, and the Puerto Rico aid alone totals more than $2 billion.