Nate Silver explains:
There is no rule of thumb about what early voting figures “should” be. The early voting advantage is presumably some function of: (i) demographics, i.e. older voters are more inclined to vote early; (ii) enthusiasm, which is peculiar to each particular election; (iii) the extent to which each party emphasizes early voting; (iv) whether or not the year is a midterm. Suppose we were trying to fit a four-variable regression model to predict early voting. You can’t really fit a four-variable model on only four data points (e.g. 2008, 2000, 2004 and 2006). It just doesn’t work, statistically. You can’t even hazard a guess.
In other words, the early voting data this year could be consistent with anything from a massive, Gallup-style Republican wave to the first sign of a major Democratic comeback. We really don’t know.
The only thing we do know, rather, is the fact cited Sunday night: in most states so far, registered Republicans are casting ballots at a somewhat higher rate than registered Democrats. Their advantage so far amounts to about 6 points, which for better or for worse, corresponds quite nicely to the roughly 6-point “enthusiasm gap” that most pollsters are seeing.
Now, it is possibly or even probably a coincidence that the turnout gap in the early voting data matches the enthusiasm gap in the polling data (a point I should have made more clearly in last night’s article). Perhaps, in a universe where the overall enthusiasm gap is 6 points in favor of Republicans, their advantage among early voters “should” be 20 points, or 2 points, or some completely different number.
We really don’t know. I mean, we really don’t know. Early voting is quite new in most parts of the country, and something most voters weren’t taking advantage of until quite recently. Perhaps by 2018 or 2020, we’ll have a better idea of how early voting operates under different sets of political conditions and will have developed better techniques for analyzing it.