Special elections are, in a word, special. They have lower turnout, there are usually only high propensity voters in the mix, and they only tell you about how things look today. So in a broader sense, we can’t, and shouldn’t, predict the future with 100 percent accuracy based on their results. But the thing is, midterms share these same characteristics. They are lower turnout elections, only higher propensity voters tend to turn out for them, and approval rates in the first year of a presidential term historically trend even lower in the second term. This means you can draw some decent correlations between special election performance by the party out of power and the midterm election results.