A toxic, racist, far-right response to Harmeet Dhillon’s Monday-night Sikh prayer at the Republican national convention is just one sign of the difficulties Donald Trump and Republicans have in selling a more diverse version of Trumpism to the party’s base. Social media posts following Dhillon’s prayer indicated that some far-right Trumpists had been polarized by the sight of a non-Christian form of religious devotion on the convention stage in Milwaukee. The barrage of hate she received from a segment of fellow Trump supporters may have been especially galling to Dhillon, whose earliest public prominence was as a civil rights lawyer...