But Leon Fresco, who defended some of the Obama administration's most controversial enforcement policies as deputy assistant attorney general for the office of immigration litigation, said Trump could win a prolonged court fight. Fresco said he'd likely lose a battle in lower courts, but that a friendlier Supreme Court could support him.
Even before the Trump-nominated Justice Brett Kavanaugh took the oath, the Supreme Court has demonstrated its willingness to give Trump wide leeway on presidential powers.
In a 5-4 ruling, the high court upheld Trump's controversial travel ban that barred nearly all travelers from five mainly Muslim countries as well as officials from North Korea and Venezuela.
And last month, the court nearly allowed the Trump administration to deny asylum to those who illegally cross the U.S.-Mexico border despite language in the law that states asylum applications from people who had entered the country unlawfully.
"Any alien who is physically present in the United States or who arrives in the United States (whether or not at a designated port of arrival and including an alien who is brought to the United States after having been interdicted in international or United States waters), irrespective of such alien's status," the statute says.
If not for Chief Justice John G. Roberts Jr., a conservative nominated by President George W. Bush, Trump would have even more leeway.
Fresco said the asylum statute is clear, but Roberts is less likely to rule against Trump on the wall.
"Why not?" Fresco said. "The question is can you name a more appropriate military construction project than to build a barrier to stop an unlawful incursion into the United States."
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The Supreme Court has never pushed back on an emergency declaration under the National Emergencies Act, according to the Brennan Center of Justice. But Waldman said there are limits to the president's executive powers.
He pointed to one of those famous instances when the Supreme Court blocked President Harry Truman from nationalizing the steel industry during the Korean War.
"So there is a history of the court standing up to this kind of thing," Waldman said. "The question for this court is will they follow in the path of courts that did stand up to presidential abuses of power or not."
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