Trump could technically close the border. Here’s why.

On the seventh day of the partial government shutdown, President Donald Trump threatened to “close the southern border entirely” if “obstructionist Democrats” refused to cave and give him funding for his border wall.

But can he do that?

“The answer is yes,” said Frank Mora, former Deputy Assistant Secretary of Defense for the Western Hemisphere, who served during President Barack Obama’s first term. “He just needs to present a kind of consultative explanation, under national security terms, why he would do this or why he needs to do this.”

Trump wouldn’t be the first president to seal off the southern border: Nixon closed sections of the border during Operation Intercept in an attempt to keep drugs from entering the U.S. So did Reagan, briefly, when a DEA agent went missing. Trump could, in theory, do the same, especially for reasons of national security, according to experts. He doesn’t even need Congressional approval.

But if the president decided to close the entire border at once, he’d almost certainly face legal challenges, similar to those that plagued the various iterations of his travel ban.

Trump took to Twitter Friday to say that if the Dems didn’t give him the $5 billion he wants to finally put up his infamous border wall, he’d shut down the border (much like the government). The president did not, however, explain what closing the border would actually mean — only that the decision would be a “profit making operation” because the “United States loses soooo much money on trade with Mexico.”

Before closing the border, Trump would need to justify the decision before Congress, although Congress can’t veto whatever he decides, according to experts. He can, however, expect to be challenged in court.

Asylum seekers have a legal right to present their case for asylum, and legal residents of the U.S. unable to get into the country due to the border closure would also likely have standing to sue. Even if Trump justified the decision through national security, his travel ban, which used similar logic, still faced an onslaught of legal challenges that prevented its implementation for months.

“It’s still an open question whether the courts have to accept the president’s claim of national security or what it would take for them to reject it,” Stephen Legomsky, who served as the Chief Counsel of U.S. Citizenship and Services in the Department of Homeland Security between 2011 and 2013, told VICE News in an email.

Friday’s Trump tweetstorm also threatened to deploy the military to the border, where governors have already sent their various National Guard troops. But the military can’t perform any domestic law enforcement functions there, according to Legomsky.

The president said he would also cut off “all aid” to Honduras, Guatemala, and El Salvador over reports of a “new Caravan” of immigrants bound for the border and accused the Central American states of of “taking advantage of U.S. for years!”

Congress, however, has already appropriated those aid funds, and the president can’t keep them from being spent. But he could urge Congress to not include aid in future spending.

READ: Meet the real power broker behind Trump’s shutdown

This isn’t the first time Trump has threatened to seal off the U.S.-Mexico border. He made a similar remark ahead of the midterm elections in response to a migrant caravan of some 7,000 that reached the U.S.-Mexico border.

In the end though, the devil would be in the details.

“Congress has created a complex, comprehensive system of criteria and processes for determining who is allowed to enter the country and who isn’t,” Legomsky said. “If the president goes too far, the courts would have to decide whether he has effectively undermined that system.”

Since July, Trump has repeatedly requested $5 billion in funding to build a wall dividing the U.S. and Mexico, which caused a partisan uproar in Congress and lead to the shutdown that affected hundreds of thousands of federal employees

Last week, however, Trump dismissed a short-term funding bill that would have delayed the fight over funds until February and instead, insisted that Congress give him money for construction right now.

READ: Government workers are sharing their #shutdownstories on social media. They’re grim.

With lawmakers trickling back into Washington from their holiday breaks and Democrats poised to soon take over their majority in the House, there’s little chance that the shutdown will end before 2019.

Cover image: File photo dated 15/3/2018 of Donald Trump, who has threatened to close the US border with Mexico if Democrats in Congress do not agree to fund the construction of a border wall. (Press Association via AP Images)