Crunching Some Numbers

According to the pooh-bah of political prognosticators, Nate Silver, Democrats’ prospects for the midterms have improved. They now have about a 71 percent chance of keeping the Senate and a 29 percent chance of holding on to the House . There's also a concern that polls might overestimate Democrats’ position in the way similar to 2016 and 2020. In other words, some of the political illuminati seem to believe that polls are biased toward Democrats.

The best version of this latter position comes from Nate Cohn, of the New York Times. He pointed out in a piece on Monday that states like Wisconsin and Ohio, where Democratic Senate candidates are outperforming FiveThirtyEight’s fundamentals index were also prone to significant polling errors in 2020. But 538's Nate offers something of a rebuttal to the idea that polling bias is predictable, and favors Democrats.

What's been dubbed the Lite version of the 538 forecast, more or less a polls-only view of the race, gives Democrats an 81 percent chance of keeping the Senate and a 41 percent chance of keeping the House. This version also suggests that Democrats will win more seats, and corresponds to current polls overstating Democrats’ position by the equivalent of 1.5 or 2 percentage points. Put another way, one can therefore think of a race in which the polling average shows Democrats 2 points ahead as being tied.

It’s also the case that in individual races, information besides the polls can help make a more accurate prediction, even when you have a lot of polls. For example, the partisan lean of a state still tells you something. Let’s say the polling average has the Democrat ahead by 10 points in a state where the fundamentals put the Republican up by 2. Empirically, the best forecast in a race like this uses a blend of mostly polls and some fundamentals. How much weight is given to the polls depends on how many polls there are, and how close it is to the election. But that aside, one might end up with a forecast that has the Democrat winning by 7 or 8 points rather than 10.

In races such as Wisconsin and Ohio, where there's a significant divergence between polls and fundamentals, Democrats probably should be concerned, and redouble or quintuple their get-out-the-vote efforts. But in contests like Pennsylvania's gubernatorial or senatorial competitions, we can breathe a cautious sigh of relief. John Fetterman currently leads Mehmet Oz by more than 9 points. Josh Shapiro leads Doug Mastriano by double digits.