MONROEVILLE—The little Alabama city looks exactly like Maycomb, that town Harper Lee imagines in To Kill a Mockingbird. The courthouse, made famous by Gregory Peck in the film adaptation of the book, still stands white in the afternoon glare. From its steps, the whole downtown is visible: insurance agencies, cafés, and the farm supply store.

And on the day of the Republican primary runoff, it’s especially easy to remember the mad dog scene from Harper’s book. The rabid animal appears. The nearly-blind Atticus Finch reluctantly drops it with one shot from his neighbor’s rifle. And the curious Scout is repeatedly told to stay away because the diseased dog is “as dangerous dead as alive.”