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WASHINGTON — As President Donald Trump has renewed his threats to shut down the southern US border, officials in his administration are nervously watching his Twitter feed.

Trump has been repeatedly warned by officials that shutting down the US-Mexico border could result in billions of dollars in economic losses, destabilize border towns and cities, and do little to stem the flow of migrants across the southern border, multiple current and former administration officials told CNN.

But faced with a surge in illegal immigration and the optics of over-capacity detention centers at the US border, Trump has returned to the possibility of taking drastic action to signal that Mexico must do more.

The results, one administration official warned, could be “catastrophic.”

“We could be in a whole world of hurt,” another White House official told CNN.

Trump has been briefed several times since last year about the economic consequences of shutting down US ports of entry at the southern border but has continued to contemplate the idea of shutting down sections or the border in its entirety. And some of his closest advisers have encouraged him to follow through.

The President has polled several of his political advisers in the last week, one source familiar with the discussions said. And while some have warned him about the economic consequences, several have relished the prospect of another dramatic move that would call attention to what they have described as an illegal immigration crisis on the southern border.

The administration dove back into discussions about shutting down the border after Trump tweeted Friday that he would be “CLOSING the Border, or large sections of the Border,” in the coming week if Mexico does not crack down on illegal border crossings.

Although the deadline was new, Trump has threatened to shut down the southern border before. This time around, White House officials have not been willing to bet whether Trump will follow through.

“You never know,” another White House official said. “It’s anyone’s guess.”

The official compared the uncertainty to the back and forth over an emergency declaration to fund construction of the border wall, which Trump ultimately decided to do.

“In his mind, he thinks that there’s leverage there,” one administration official said, explaining that Trump believes shutting down the border would pressure Mexico to stop more of the migrants who hail from Central America from traveling through the country to cross into the US.

White House officials have held several meetings since last year to discuss the possibility of shutting down the US-Mexico border.

During a November meeting helmed by the President’s Domestic Policy Council staff, officials began to consider the economic impacts of various scenarios, including a shutdown of all US-Mexico ports of entry versus shutting down only the ones linking Mexico to California or Texas, one administration official said. The officials also worked to assess the economic impact on Mexico.

While Trump’s threats to close the border have come and gone, some aides believe he is now more likely to turn that rhetoric into action. He is surrounded by fewer aides who view their role as guarding against his most impulsive decisions and is being encouraged by several top advisers to press his hardline agenda on immigration.

A move this week to seal off parts of the US-Mexico border would come on the heels of a decision to cut off foreign aid to several Central American countries, another move he had repeatedly threatened since early in his presidency but acted on only last week.

It is the third time Trump has threatened to close the border.

“This time seems more real,” one administration official said.

‘We’ll be looking into’ economic impact

Earlier Monday morning, a top White House economic adviser declined to specify what economic impacts the administration expects if Trump closes the US-Mexico border this week.

“It’s something that I’m sure we’ll be looking into and studying,” Kevin Hassett, the chairman of the Council of Economic Advisers, told reporters Monday.

The economic impact would likely be significant if Trump follows through. The US and Mexico are major trading partners, and along with Canada, joined together last year for a ceremonial signing of the US-Mexico-Canada Agreement, a potential replacement for NAFTA.

Trump threatened on several occasions last year to close the US-Mexico border, including at one point saying the US would close the border “permanently.”

“Mexico should move the flag waving Migrants, many of whom are stone cold criminals, back to their countries. Do it by plane, do it by bus, do it anyway you want, but they are NOT coming into the U.S.A. We will close the Border permanently if need be. Congress, fund the WALL!” he tweeted in November.

Hassett spoke to reporters outside the White House on Monday morning, and when asked if there was economic modeling to gauge the potential economic impact of a shutdown at the border, he stressed the complex and unique nature of the issue.

“There’s a lot of stuff that moves and exactly how Customs and Border Patrol would deal with that would be something that we’ll have to confer with them about, should we get to that,” Hassett said.

He added, “There would be potentially more stuff sourced in the US for a while, but it’s something that we’d have to talk to Customs and Border Patrol about.”

Hassett noted that there could be “special rules for certain types of things” in the event of a shutdown at the border.

He also referenced the Bush administration’s approach to the border in the wake of the September 11, 2001, attacks. The US response to September 11 included increased inspections and led to longer lines at the border, and it came alongside a temporary air travel shutdown.

Legal experts have been divided on the extent to which Trump could “close” the border.

CNN legal contributor Steve Vladeck said last year that if Trump made the move, he could not “do it in a way that violates constitutional rights — whether of those of a particular national origin, or of those non-citizens with sufficient connections to the US to trigger due process protections.”

“For instance, categorically closing the border between El Paso and Ciudad Juarez would trap thousands (if not tens of thousands) of Americans and non-citizens lawfully entitled to be in the US on the other side of the border, even though many of them would have a strong claim of a right to travel / return home / etc. that could raise serious constitutional problems,” said Vladeck.