Psephologists—the fancy word for election analysts—like to talk about change and transformation. One party or the other, they say, is on the verge of forging an enduring national majority. One party or the other is doomed.

What I have come to see in my number crunching is not change and transformation, but continuity. Three presidents in a row have been re-elected with 49%, 51% and 51% of the vote. Over the past two ...