The Political polling analysis site, FiveThirtyEight, is now projecting a possible loss of fifty congressional seats for Democrats. Most pollsters have been projecting that Republicans may take back the House of Representatives but not a greater majority that Democrats enjoy.
538 puts it this way:
A couple of weeks ago, we examined the potential upside case for Democrats in November’s midterms. If the party were able to limit their losses to about 20 House seats and 3-4 Senate seats, it might not have as deleterious an effect on their policy agenda as you might think.
But that is the upside case for Democrats. It is not the base case, and it is certainly not the worst case — both of which look as grim as ever. Although I think people may somewhat underestimate the probability of a shift in momentum back toward the Democrats, they may simultaneously be underestimating the magnitude of losses that might occur if momentum fails to change, or moves in the other direction.
For starters, let’s look at the state of the generic congressional ballot. The Real Clear Politics average now shows Republicans with a 2.3 point lead. How does that translate in terms of a potential loss of seats for the Democrats?
Let’s suppose for a moment that, in November, the Democrats lose the national house popular vote by a margin of 2.3 points. It is actually not safe to assume that a 2.3-point deficit in generic ballot polls translates to a 2.3-point loss in the House popular vote — but we’ll get to that ambiguity in a moment.
You might assume that, if a party loses the popular vote in the House, they are unlikely to retain control of the chamber. But it is not quite that simple. For reasons that are somewhat debated, the number of seats that a party controls going into the Congressional election still has predictive power in forecasting the seat count that emerges, even if we take as a given the results of the national popular vote. In particular, the larger a party’s majority going in, the more seats they tend to add or retain relative to their performance in the popular vote.