Even as Donald J. Trump trounced him from New Hampshire to Florida to Arizona, Senator Cruz could reassure himself with one crucial advantage: He was beating Trump in the obscure, internecine delegate fights that could end up deciding the Republican nomination for president. “This is how elections are won in America,” Cruz gloated after walking away with the most delegates in Wyoming. But it turns out that delegates — like ordinary voters — are susceptible to shifts in public opinion. As the gravitational pull of Trump’s recent primary landslides draws more Republicans toward him, Cruz’s support among the convention delegates...