ST. ROBERT, Mo.—Claire McCaskill began a sparsely attended campaign event here Thursday with this quip: "I don't know if you noticed, but my campaign has been really boring lately." The ironic line was one of the few references the Democratic senator made to the political firestorm that erupted after her opponent, U.S. Rep. Todd Akin, used the term "legitimate rape" during a TV interview Sunday. Rather than beat Mr. Akin over the head, Ms. McCaskill appears to have decided to let the wrecking ball he set in motion swing all by itself. And unlike the Republican establishment, which has called...