Acting White House Chief of Staff Mick Mulvaney said on Sunday that “something dramatic” would have to happen for President Trump to not follow through with his vow to shut down the United States’ southern border.

“When Jeh Johnson said it’s a crisis, I hope people now believe us,” Mulvaney said during an interview on ABC’s “This Week.” “Democrats didn’t believe us a month ago, two months ago, when we said what was happening at the border was a crisis: a humanitarian crisis, a security crisis.”

Mulvaney added: “100,000 people coming across the border this month… that is a crisis.”

Mulvaney’s comments come just days after Trump declared he is likely to shut down America's southern border next week unless Mexican authorities immediately halt all illegal immigration.

TRUMP THREATENS TO CLOSE BORDER 'NEXT WEEK' IF MEXICO DOESN'T 'IMMEDIATELY STOP' FLOOD OF ILLEGAL IMMIGRANTS

Trump has been promising for more than two years to build a long, impenetrable wall along the border to stop illegal immigration, though Congress has been reluctant to provide the money he needs. In the meantime, he has repeatedly threatened to close the border, but this time, with a new surge of migrants heading north, he gave a definite timetable.

A substantial closure could have an especially heavy impact on cross-border communities from San Diego to South Texas, as well as supermarkets that sell Mexican produce, factories that rely on imported parts, and other businesses across the U.S.

The U.S. and Mexico trade about $1.7 billion in goods daily, according to the U.S. Chamber of Commerce, which said closing the border would be "an unmitigated economic debacle" that would threaten 5 million American jobs.

Trump tweeted Friday morning, "If Mexico doesn't immediately stop ALL illegal immigration coming into the United States through our Southern Border, I will be CLOSING the Border, or large sections of the Border, next week."

Trump's latest declaration came after Mexican President Andres Manuel Lopez Obrador said his country was doing its part to fight migrant smuggling. Criminal networks charge thousands of dollars a person to move migrants through Mexico, increasingly in large groups toward remote sections of the border.

"We want to have a good relationship with the government of the United States," Lopez Obrador said Friday. He added: "We are going to continue helping so that the migratory flow, those who pass through our country, do so according to the law, in an orderly way."

HOUSE DEMS FAIL TO OVERRIDE TRUMP VETO IN FIGHT OVER BORDER EMERGENCY DECLARATION

Trump also moved Saturday to cut direct aid to El Salvador, Guatemala and Honduras, whose citizens are fleeing north and overwhelming U.S. resources at the border.

The State Department notified Congress that it would look to suspend 2017 and 2018 payments to the trio of nations, which have been home to some of the migrant caravans that have marched through Mexico to the U.S. border.

Sen. Bob Menendez, D-N.J., ranking member of the Senate Foreign Relations Committee, spoke out Saturday against cutting off aid to Central America, declaring that "foreign assistance is not charity; it advances our strategic interests and funds initiatives that protect American citizens."

And a group of House Democrats visiting El Salvador denounced the administration's decision to cut aid to the region.

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"As we visit El Salvador evaluating the importance of U.S. assistance to Central America to address the root causes of family and child migration, we are extremely disappointed to learn that President Trump intends to cut off aid to the region," said the statement from five lawmakers, including Rep. Eliot L. Engel of New York, who chairs the House Foreign Affairs Committee. "The president's approach is entirely counterproductive."

The Trump administration has threatened before to scale back or cut off U.S. assistance to Central America. Congress has not approved most of those proposed cuts, however, and a report this year by the Congressional Research Service said any change in that funding would depend on what Congress does.

The Associated Press contributed to this report.