Many in the Central American countries known as the Northern Triangle — El Salvador, Guatemala and Honduras — appear to be doing just that. Those nations have accounted for many of the undocumented immigrants who have tried to cross the American border in recent years. Now the wariness about Mr. Trump’s immigration policies is palpable, the impact visible.

Migrant smugglers in Honduras say their business has dried up since Mr. Trump took office. Fewer buses have been leaving the northern Honduran city of San Pedro Sula bound for the border with Guatemala, the usual route for Honduran migrants heading overland to the United States. In hotels and shelters along the migrant trail, once-occupied beds go empty night after night.

Marcos, a migrant smuggler based near San Pedro Sula, said that last year he had taken one or two groups each month from Honduras to the United States border. Since Mr. Trump’s inauguration, however, he has had only one client. He blames Mr. Trump.