No-one doubts that Cruz, a former national college debating champion, is exceptionally bright. Most exceptionally bright people cannot resist the temptation to wing it, simply because they can. Not so Cruz. He combines high intelligence with enormous discipline and capacity for work. Betsy McCaughey, a former Republican Lieutenant-Governor of New York, observes that self-discipline of this kind bespeaks humility: he is smart enough to know that even smart people can make big mistakes, and that hard work is the best insurance against missteps. Humility is not a personality trait one immediately associates with a politician who has opposed virtually all...