I made 29 predictions last January, and since accountability matters, how’d I do? At year’s end, I got 19½ right and 9½ wrong—in some instances, wildly so.

Begin with the correct predictions: The GOP modestly expanded its Senate ranks, picking up two seats, and dropped below 30 governorships, their tally falling to 27. Republicans also shed several hundred state legislative seats: 253 to be precise.

The...