Princeton Election Consortium

Innovations in democracy since 2004

Oct 25: Biden 360 EV (D+5.6% from toss-up), Senate 52 D, 48 R (D+4.2%), House control D+5.0%
Moneyball states: President ME-2 NV NC, Senate AK MT IA, Legislatures KS TX NC

Election tracking 2020: U.S. House of Representatives

Despite the complexity of having 435 House races, a single survey quantity does a good job of tracking the House: the generic Congressional ballot (“do you support the Democrat or Republican for your local Congressional race?”). There are two reasons supporting this. First, districts are of equal population, and a national survey only has to weight them all equally. The first-past-the-post (plurality vote-getter wins) rule makes this simple. Second, regional differences average out in terms of how votes translate into number of seats won.

The black trace is a polling median. Its “averaging” rule is to take an N-week median of polls, one poll per pollster, the date defined by the last day that the poll was being conducted. For our House tracker, N=3.

The next task is to convert this voting tendency to representation. We’ve opted to take an approach that reflects (a) the complexity of representation in the U.S., and (b) the fact that in the House, the exact size of the majority does not matter that much.

For these reasons, we focus on where national sentiment is, compared with the threshold necessary to gain control of the chamber. The resulting measure is in units of popular opinion. If you want to convert the meta-margin to a seat margin, the conversion factor has been about 6 in the past. For example, a meta-margin of D+7% would correspond to an approximate 42-seat margin, i.e. around 239-197.

In the U.S. representation is strongly dependent on how districts are drawn. Because districts have to be contiguous, they are constrained by how voters arrange themselves.┬áVoters favor Democrats by 30 points in urban areas, and Republicans by 15 points in rural areas. That 30%-vs-15% asymmetry creates a natural tendency for Democrats to be packed into fewer districts. It also provides the raw material for drawing lines to place them at a further disadvantage. In other words, natural geography is the starting point for partisan gerrymandering that we’ve seen explode since 2000.

We had to estimate what it would take for partisan control to switch between the parties. In 2018, I estimated the Democrats needed to win the popular vote by 6% (“D+6%”). This year, because of state court decisions in Pennsylvania and North Carolina, and because Democrats have more incumbents, I estimate that the threshold is lowered to D+3%. I base this on comparison with the historical pattern since 1948, plus analysis of current single-party gerrymanders. Think of it as analogous to a golf handicap, where the abstract ideal of majoritarian rule collides with the tradition of drawing districts.

This handicap is included in the graph above. The right axis shows the actual polling median, and the left-hand axis is the same median shifted by 3 percentage points, to show the actual advantage in terms of representation.

The November prediction is generated from current conditions, combined with a prior based on real election outcomes: special elections. For the last few cycles, special elections have been a good predictor of how the national vote will go; for example, see the 2018 election. A prediction is generated by assuming (a) random drift from the snapshot of current conditions (the black trace), and (b) combining it with a distribution of likely outcomes based on special elections (the “prior”).
That combination generates a hurricane-strike-zone-like prediction: a red zone where about two-thirds of outcomes fall (one sigma), and a yellow zone where 95% of outcomes fall (two sigma).

Because I have set the one-sigma uncertainty on current polls as having a minimum value of 3 points, as November approaches, the prior will keep on influencing the prediction, and pull it towards D+6% popular vote (D+3% above threshold for retaining Democratic control). We will see which ends up coming closer.

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Can we as citizens get rid of the handicap? An open question in our democracy is whether the “handicap” above can be made close to zero, achieving similar treatment of the major parties, while still observing traditional districting principles. Achieving that outcome requires bipartisan or non-partisan control over redistricting. Another question is whether candidates of either party have to work for re-election, or are protected by a well-designed map. This is done best by independent commissions. To learn more about how to achieve those goals, see our work at gerrymander.princeton.edu.