Jeb Bush went to the border town of McAllen, Tex., on Monday to raise money and to talk about immigration, in English and fluent Spanish. Because the Republican presidential campaign has been so fixated on border security and the immigrant peril — thank you, Donald Trump — it was a chance to see how the supposed expert on this fraught subject handled it. Short version: He was awful. In less than 15 minutes, Mr. Bush managed to step on his message, to give Mr. Trump a boost and to offend Asian-Americans, a growing population that is every bit as important...