U.S. Vice President Mike Pence reiterated Monday that the White House is looking into whether President Donald Trump can declare a national emergency at the U.S.-Mexico border that would allow him to order the military to build a wall.

Trump said Tuesday afternoon on Twitter that he will give a speech Tuesday at 9 p.m. ET “on the Humanitarian and National Security crisis on our Southern Border.” Some are speculating he will declare a national emergency in the speech.

• Watch: Explaining Trump's border wall

But Yale Law Professor Bruce Ackerman tells CTV News Channel that there is no legal way for Trump to use such a decree to build his border wall.

Ackerman points to a U.S. Supreme Court case decided when President Harry Truman nationalized steel mills and ordered all striking employees back to work during the Korean War, in violation of federal law.

“The Supreme Court holds that this is unconstitutional to use the Commander in Chief power to violate federal domestic law,” according to Ackerman. “That is precisely what President Trump is proposing to do.”

Ackerman also points to a 1956 statute that states: “Whoever, except in cases and under circumstances expressly authorized by the Constitution or Act of Congress, willfully uses any part of the Army or the Air Force ... to execute the laws shall be fined under this title or imprisoned not more than two years, or both.”

“This means that, immediately, members of the army are going to have to make a hard choice,” says Ackerman. “Should they obey the president and commit a criminal offence or should they uphold the rule of law and disobey the president?”

“This is a tragedy,” Ackerman went on. “The crisis of this kind in which the military has to decide whether it’s going to uphold the law or obey the president is precisely the scenario that has led many Latin American countries to military coups for the last 150 years.”

“Worse yet, he’s violating the Constitution in a way that requires each military man to decide whether he’s going to obey the Constitution as he is sworn to do in his oath or the president of the United States,” Ackerman added.

Earlier this week, Ackerman outlined a number of reasons in an op-ed for The New York Times about why he’s certain Trump can’t legally order the military to build his wall by declaring a national emergency.

In addition to it not being constitutional and violating the 1956 statute, he writes that it’s “possible to imagine a situation in which the president might take advantage of the most recent exception, enacted in 2011, which authorized the military detention of suspected terrorists associated with Al Qaeda or the Taliban,” but goes on to say that would be “unconscionable.”

Trump did claim on Friday that there are “terrorists coming through the southern border” and White House Press Secretary Sarah Huckabee Sanders said on Fox News last week that “nearly 4,000 known or suspected terrorists” have come into the country illegally, mostly from Mexico. A State Department report issued in September found "no credible evidence indicating that international terrorist groups have established bases in Mexico, worked with Mexican drug cartels or sent operatives via Mexico into the United States."

With files from The Associated Press