... [I]n a campaign that has seemingly tested every political rule, Mr. Trump’s opponents hope his latest provocation will be too much for well-mannered voters in this heavily evangelical Christian state to bear: his use of a pungently vulgar word this week to describe one of his rivals, Senator Ted Cruz. Mr. Trump’s raunchy language has become unsurprising at his rallies. And the slur against Mr. Cruz was largely overshadowed by the coverage of the next day’s New Hampshire primary. But in voicing the crude term, Mr. Trump has further polarized a Republican Party already deeply divided about his candidacy,...