From conversations locally, it seems that a fair number of rank-and-file supporters of President Donald J. Trump really fear, deep in their hearts, the prospect of a terror attack within the United States by radicalized Muslims. As irrationally improbable as that may be — deadly violence in the United States since 9/11 is overwhelminly a homegrown crisis — the so-called immigration ban makes them at least feel safer. They are not alone; according to polls cited by The New York Times, almost half of U.S. respondents favored more restrictions on migrants from “terror-prone” regions.