Not so long ago, Hillary Clinton’s supporters main concern seemed to be a fear that her coronation as the Democratic candidate for president would leave her unprepared for battle with the Republican nominee. Now, by all metrics, the former secretary of state retains a historically strong lead in the race to secure her party’s nomination. She is well ahead of the other declared candidates in terms of poll numbers, money and endorsements. But a succession of setbacks and the possibility of another mainstream rival joining the race has, to some degree, checked the presumptiveness of the presumptive candidate. This was...