Projected 2018 Democratic U.S. House seat gains

This piece (only available to subscribers) appeared earlier today on Taegan Goddard’s absolutely essential Political Wire.

A new Brennan Center report says “extreme gerrymandering” could cost Democrats control of the House unless they ride a massive blue wave.

Because of maps designed to favor Republicans, Democrats would need to win by a nearly unprecedented nationwide margin in 2018 to gain control of the House of Representatives. To attain a bare majority, Democrats would likely have to win the national popular vote by nearly 11 points. Neither Democrats nor Republicans have won by such an overwhelming margin in decades. Even a strong blue wave would crash against a wall of gerrymandered maps.

Yet this is misleading without also mentioning the “the great sorting” of voters that has taken place over the last two decades. An equal, if not bigger, barrier to Democrats winning the House is the extreme urbanization of Democratic voters which leads to millions of “wasted” votes.

 Pew Research study shows:

Voters in urban counties have long aligned more with the Democratic Party than the Republican Party, and this Democratic advantage has grown over time. Today, twice as many urban voters identify as Democrats or lean Democratic (62%) as affiliate with the GOP or lean Republican.

Overall, those who live in suburban counties are about evenly divided in their partisan loyalties (47% Democratic, 45% Republican), little changed over the last two decades.

In addition, while mapping technology has made it easier for congressional maps to be gerrymandered in the redistricting process, the sorting of the electorate into distinct geographic areas makes it even easier.

Both phenomena — the use of gerrymandering during redistricting and the geographic sorting of voters — coexist and give Republicans an advantage in congressional elections.

What does this mean for the 2018 midterm elections? The Cook Political Report has found that in the last three election cycles, Democrats have won roughly 4% fewer seats than votes received nationally. If this trend holds true in 2018, then Democrats would need to win the House popular vote by roughly 7% to win the 23 seats they need to take a majority. This is a similar to a projection made by Emory University political scientist Alan Abramowitz.

In contrast, the Brennan study, which looks at responsiveness of vote margins in individual states, suggests Democrats would need an 11% margin to take control of the House.

I have expressed mild skepticism about the role gerrymandering has played in the maintenance of Republican majorities in the United States House of Representatives (House) since 2011, writing this about “wasted” votes (which I call “extraneous votes” [ExV]):

In 2016, Democrats averaged 112,222 ExV and Republicans averaged 98,582, meaning Democrats averaged 13.8% more ExV than Republicans. Narrowing the analysis only to the 39 states where partisan redistricting is even possible closes the gap: 111,401 to 102,963, with Democrats averaging 8.2% more ExV. Further removing seats with candidate(s) of only one major party reduces the absolute gap to 97,701 to 89,970, with Democrats averaging 8.9% more ExV than Republicans.

This is additional evidence for the geographic self-sorting of Democrats, which I agree (along with the creation of majority-minority legislative districts under the Voting Rights Act) has enabled Republican gerrymandering. I would also observe that Republicans won a net total of 63 House seats (and House control) in 2010, before the current legislative district lines were drawn.

The piece concluded with a tabulation of projected 2018 Democratic net House seat gains given a range of Democrats national House vote margins using the “Abramson” and “Brennan” models. Democrats need a net gain of 23 House seats to recapture the majority.

Given my own research into the relationship between national House vote margin and House seats won (using change in Democratic share of the national House vote from two years earlier; Democrats lost the national House vote by 1.1 percentage points in 2016, despite netting six seats), I decided to append my projections to Goddard’s table. “Berger 1” uses percentage point change only, while “Berger 2” also adjusts for midterm vs. presidential election. For my projections, I display both the estimated net seat gain as well as the probability Democrats net the 23 seats necessary to regain House control. In each column, boldfaced values represent Democratic House control.

Table 1: Projections of 2018 Democratic net gains in House seats

Dem national House margin Abramson Brennan Berger 1 Berger 2
2% 19 5 8




4% 23 7 14




6% 27 13 20




8% 30 15 27




10% 34 21 33




11% 36 28 36




12% 38 31 39




14% 42 41 45




16% 46 56 52




My projections fall in between those of Abramson and Brennan: Berger 1 requires Democrats to win the national vote by 6.8 percentage points, while Berger 2 requires a margin of only 5.4 percentage points (see Figure 1 below).

Figure 1: Probability Democrats Control U.S. House of Representatives After 2018 Elections Based Upon Change in Democratic National House Vote Share, 2016-18

Democratic Probability 2018 House capture

As of this writing (7:24 pm EST, March 26, 2018), the FiveThirtyEight estimate of Democratic advantage in the generic ballot is 5.7 percentage points (46.0 to 40.3%, down from a high of 13.3 percentage points on December 26, 2017). If that is the actual national House vote margin on November 6, 2018, Democrats would be projected to net between 12 and 27 House seats, depending on the model, meaning they either fell well short of their goal, or just eked it out.

Still, it is a sign of the current lopsided state of American electoral geography that a political party needs to win the national vote by between 4 and 11 percentage points just to break even in House seats.

Until next time…

4 thoughts on “Projected 2018 Democratic U.S. House seat gains

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