Donald Trump has a new threat about the U.S.-Mexico border: If he doesn’t get his way, he might just shut down the whole thing.

The president appeared to contradict a deal his administration reached with the Mexican government under which allow asylum seekers could remain in Mexcio as a legal process about their request to enter the United States played out. But on Monday morning, Trump pressed Mexican officials to “move the flag waving Migrants, many of whom are stone cold criminals, back to their countries.”

He appeared to be referring to several groups of Central Americans who have been traveling by foot from their home countries toward the U.S.-Mexico border. Trump used the so-called “caravans” as a campaign trail talking point during the final weeks of the midterm elections; the issue was less of one for him and conservative media for a few weeks after Election Day, but they have returned to it in the last week or so.

Of those migrants, Trump tweeted Monday a vow that “they are NOT coming into the U.S.A. We will close the Border permanently if need be.”

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!

— Donald J. Trump (@realDonaldTrump) November 26, 2018

It is doubtful that the U.S. government has the personnel or funding to close the entire 1,954-mile border, experts have said. The president, as he often does, excluded any specific plans for how he would seal the border for the remainder of time, a personnel- and resource-intensive task.

Democratic lawmakers on Sunday slammed Trump’s border-closing bravado as illegal.

“That’s not the law,” incoming House Government and Oversight Chairman Elijah Cummings, D-Md., told NBC’s “Meet the Press.”

“They should be allowed to come in, seek asylum, that’s the law,” Cummings said. “I think we have a system that has worked for a long time. This president’s come in, wants to change it, that’s up to him. But now the Congress has got to stand up and hopefully they will.”

Sen. Amy Klobuchar, D-Minn., a potential 2020 presidential candidate, told ABC’s “This Week” that the president “should have been working with these Central American countries a long time ago to try to get to a point where we didn’t see this extraordinary amount of people coming through.”

She said Trump is the biggest impediment to Congress passing a sweeping border security bill.

“He has gut punched us on that a number of times. We have the will to put the money at the border for better security and combine it with some sensible reforms, including things like a path to citizenship, things like making sure that we have workers on our fields and in our factories that we need,” Klobuchar said. “But he has chosen instead to weaponize this, to politicize it.”

Rep. David Cicilline, D-R.I., tweeted his concerns on Sunday about reports of border patrol personnel using tear gas on migrants, while Hawaii Democratic Sen. Brian Schatz wondered, in all capital letters, where that order came from.

Border Patrol firing tear gas on asylum seekers, including kids, at the border. This is not America.

— David Cicilline (@davidcicilline) November 25, 2018

WHO GAVE THE ORDER?

— Brian Schatz (@brianschatz) November 25, 2018

And Maxine Waters, D-Calif., who has traded rhetorical jabs with Trump, called his border and migrants talk a “political ploy.”

“He certainly did stoke a lot of fear about these migrants who are trying to get in,” she told CNN. “Now, we have this chaos. That’s what he wants. … He’s not interested in credible reform.”

Watch: Border Wall Funding Fight Could Lead to Another CR, or Partial Shutdown