US President Donald Trump has talked of a growing humanitarian crisis at the southern border and has called on Congress to fund the wall.
But in his first TV address to the nation from the Oval Office, Trump did not declare a national emergency to bypass Congress and build the barrier.
“All Americans are hurt by uncontrolled illegal migration,” Trump said, speaking from the White House.
“This barrier is absolutely critical to border security. This is just common sense. It would very quickly pay for itself.”
Trump rattled off a number of grisly murders by illegal immigrants to justify the funding.
“How much more American blood must we shed before congress does its job?” he said.
“This is a choice between right and wrong – justice and injustice.”
A partial government shutdown has been in effect for 17 days after lawmakers failed to break a budget impasse.
The president is insisting that $US5 billion ($NZ7.4b) be included for the border wall.
Following Mr Trump’s address, House Speaker Nancy Pelosi said he was holding the American people hostage over the wall and was using fear to garner support for his message.
Senate Minority Leader Chuck Schumer also accused Trump of trying to “govern by temper tantrum” and of manufacturing a crisis.
“President Trump has appealed to fear, not facts. Division, not unity,” the New York senator added.
He concluded: “The symbol of America should be the Statue of Liberty, not a 30-foot wall.”
Trump is scheduled to visit the southwest border on Thursday and it was not clear whether he still might choose to make the national emergency declaration.
Though both Democrats and Republicans agree there is a crisis at the border, critics have accused Trump of greatly exaggerating the problem.
The number of illegal border crossings is down from 1.6 million in 2000 to fewer than 400,000 last year.
Research also indicates that undocumented immigrants are much less likely to commit crime than native-born American citizens.
The White House suggested at the weekend that thousands of terrorists were caught attempting to cross the US-Mexico border, but in fact all but a handful were stopped at airports.