For instance, 27 percent of Americans endorsed violence against the government under some circumstances in a 1998 Pew poll — the same accusation he made against American Muslims. More recent surveys conducted by Nathan Kalmoe of Monmouth College indicate that support for anti-government violence extends for millions of Americans to specific actions such as sending threats to politicians who are “damaging the country” (9 percent), throwing a brick through the window of the “worst politicians” (6 to 13 percent) and fixing “some of the problems citizens have with government” with “a few well-aimed bullets” (5 to 6 percent).

Americans also frequently wish to remove legal protections from certain groups, which again echoes Mr. Trump’s fear-mongering about imposing Shariah law. James Gibson at Washington University in St. Louis found in a 2005 survey, for example, that more than half of Americans endorsed unconstitutional limitations on speech by the groups they liked least, which included widespread support for banning demonstrations by “radical Muslims” (57 percent), atheists (47 percent) and religious fundamentalists (39 percent).

Given these findings, Mr. Trump’s turn toward extremism can be understood as a result of his willingness to exploit existing undercurrents in public opinion.