“Mexico is tough,” Mr. Trump said. “If they don’t stop them, we’re closing the border.”
Americans would feel the effects in other ways. Border control agencies are already reviewing ways to slow down immigration processing at the border. A senior homeland security official confirmed on Friday that shutting down ports of entry along the southwest border was “on the table” to handle the surge in migrants seeking asylum.
Kirstjen Nielsen, the homeland security secretary, said in a statement 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.
“Make no mistake: Americans may feel effects from this emergency,” Ms. Nielsen said. “As personnel are reallocated to join the crisis-response effort, there may be commercial delays, higher vehicle wait times at the border and longer pedestrian lines.”
She added that “despite these impacts, we cannot shirk our responsibility to the American people to do everything possible to secure our country while also upholding our humanitarian values.”
Stephen H. Legomsky, a law professor at Washington University in St. Louis, said there were a few options available to Mr. Trump if he wanted to move to stop the flow of people at the border, including slowing down processing at ports of entry. The administration has already limited the number of migrants who can apply for asylum each day.
“He could pretend that this was necessary in the interest of national security,” Mr. Legomsky said, “and it could be that the courts would be hesitant to second guess a national security claim by a president.”
Mr. Legomsky said putting up trade roadblocks and slowing down the processing of people would have harmful effects, both on American citizens who could find themselves “marooned” in Mexico and on families, including groups of women and children, who are fleeing violence and poverty.