A full or partial sealing of the border would effectively close off the United States from one of its largest trading partners, and it could leave American citizens who cross back and forth with a sluggish or potentially nonexistent system of returning to the United States. Mr. Trump will travel to Calexico, Calif., to tour the border on Friday.

Kirstjen Nielsen, the homeland security secretary, said in a statement on Friday that she had asked volunteers to add more support at the border, and suggested that American citizens may encounter difficulty getting through as a result.

And by directing the State Department to revoke aid from the three Northern Triangle countries, Mr. Trump is seeking to punish those countries for what he says is a failure to stop migrant caravans from making their way north.

Leaders of those countries have said they are committed to solving the problem, despite what Mr. Trump has said. In 2017, the United States gave $420 million to Central American countries struggling with violence. According to the U.S. Global Leadership Coalition, a nonprofit organization that supports diplomatic efforts of the United States, the Northern Triangle countries committed about ten times that amount, or $5.4 billion, to improve conditions.

“American aid to Central America is not charity, but an investment in our national security,” Liz Schrayer, the president of the coalition, said in a statement. “The idea of suspending the relatively small, but essential foreign assistance to the region will only exacerbate the root causes driving people to flee their homes — brutal violence, hunger and instability.”

In an interview with Jake Tapper on CNN’s “State of the Union,” who pointed out to him that experts within the president’s own administration have said aid money has helped curb violence in and migration from El Salvador, Mr. Mulvaney shrugged off the data from “career staffers” and said the money had not done enough.