Senator JD Vance, the running mate of former President Donald J. Trump, has declined to denounce the right-wing talk-show host Tucker Carlson for praising and airing the views of a Holocaust revisionist who falsely claimed that the Nazis’ destruction of European Jewry was not an intentional act of premeditated genocide. Mr. Vance is scheduled to appear onstage Sept. 21 with Mr. Carlson in Hershey, Pa. Mr. Carlson no stranger to controversy, but his recent interview with Darryl Cooper, whom he described as “the best and most honest popular historian in the United States,” has faced particularly fierce blowback. The Nazis’...