The National Emergencies Act is intended for times of national crisis. Mr Trump says there is a migration crisis at the nation’s southern border – a claim strongly refuted by migration experts.
The largest number of illegal migrants settling in the US each year is those who stay in the country after their visas expire.
Declaring a national emergency would give the president access to special powers that effectively allow him to bypass the usual political process, and he would be able to divert money from existing military or disaster relief budgets to pay for the wall.
However, critics accused the Trump administration of a “manufactured crisis”, saying the president’s assertions about the situation were untrue and thathe misused his powers to declare emergency.