Father and son walk in a caravan of migrants en route to U.S. in San Salvador

A father and son walk in a caravan of migrants departing from El Salvador en route to the United States, in San Salvador March 30, 2019.

WASHINGTON — The Trump administration on Sunday doubled down on its threat to shut down the southern border with Mexico, a day after it cut aid to Central American countries that President Donald Trump accused of deliberately sending migrants to the United States.

Faced with a surge of asylum seekers from Central American countries who travel through Mexico, Trump said on Friday there was a “good likelihood” he would close the border this coming week if Mexico does not stop unauthorized immigrants from reaching the United States.

Without providing evidence, he also accused the nations of having “set up” migrant caravans and sending them north.

Speaking to ABC’s “This Week” show, White House acting Chief of Staff Mick Mulvaney said the President had few other options in the absence of any support from Democrats for more border security or legislative action to change the immigration law.

“Faced with those limitations, the President will do everything he can. If closing the ports of entry means that, that’s exactly what he intends to do,” Mulvaney said. “We need border security and we’re going to do the best we can with what we have,” he added.

White House adviser Kellyanne Conway told “Fox News Sunday” that the situation at the border was at “melting point” and said the President was serious in his threat. “It certainly is not a bluff. You can take the President seriously.”

Neither Trump aide offered any specific details or timeline for the potential border shutdown.

Trump has repeatedly said he would close the U.S. border with Mexico during his two years in office.

His latest threat had workers and students who frequently cross the border worried about the potential disruption to their lives.

The government says it is struggling to deal with a surge in recent days of asylum seekers from countries in Central America who travel through Mexico and on Saturday cut aid to El Salvador, Guatemala and Honduras — a move other Democrats warned would only worsen the situation.

“What we need to do is focus on what’s happening in Central America, where three countries are disassembling before our eyes and people are desperately coming to the United States. The President’s cutting off aid to these countries will not solve that problem,” Senate minority whip Dick Durbin told NBC News’ “Meet the Press.”

He also cast doubt on the viability of shutting the border, describing the threat as “totally unrealistic.”

March is on track for 100,000 border apprehensions, Department of Homeland Security officials said, which would be the highest monthly number in more than a decade. Most of those people can remain in the United States while their asylum claims are processed, which can take years because of ballooning immigration court backlogs.

Trump is expected to visit the border within the next two weeks.

While some Trump critics say he is bluffing, closing the southern border is not unprecedented. Presidents Richard Nixon and Ronald Reagan both shut the border over drug-related issues, while President Lyndon B. Johnson closed it briefly after the assassination of John F. Kennedy in 1963.

Trump could turn to the section of immigration law that grants the President broad authority to prevent certain people from entering the country on national security grounds, legal experts said. That authority was upheld by the Supreme Court in a decision last year on his controversial travel ban.

But it is unclear what powers Trump would use to invoke that law.

If he were to declare a national emergency, he could face difficulty justifying that the influx of asylum seekers amounts to a “true and legally demonstrable federal ‘emergency’” and warrants the “extreme measure of closure,” said Geoffrey Hoffman, the director of the University of Houston Law Center Immigration Clinic.

Shutting the border would likely trigger other legal challenges, including by asylum seekers whose rights to seek entry were upheld by a Ninth Circuit Court of Appeals December ruling.

“Many of the Central American families arriving in the U.S. are asylum seekers and claim a fear of return,” said Lindsay Harris, a professor at the University of the District of Columbia’s law school and co-director at its Immigration and Human Rights Clinic. “This fear is warranted.”