What's the difference between a calm and cool Barack Obama, and a rattled and worried Barack Obama? Four words, it turns out. "You didn't build that" is swelling to such heights that it has the president somewhere unprecedented: on defense. Mr. Obama has felt compelled—for the first time in this campaign—to cut an ad in which he directly responds to the criticisms of his now-infamous speech, complaining his opponents took his words "out of context."