But, because Democrats are clustered in cities and face harsh gerrymanders , they aren’t expected to win an equivalent share of the seats in Congress. What does electoral geography tell us about the actual outcome?

In my projection of the Election Day vote share, based on polls of the generic ballot and the swing toward Democrats in special elections, The Democratic Party is ahead, winning by 8.8% of the vote share on average. The margin of error is roughly 6% points. Below are the generic congressional ballot polls used to make that projection:

My forecast of the election day vote works in three stages. First, I average all of the generic ballot polls with an algorithm designed to produce the most predictive average for each week in the cycle. Second, I use that average to predict the most likely election day polling average for Republicans, Democrats, other parties and undecided voters. Finally, I combine the projected Democratic margin in election day polls with Democrats’ average performance in special elections between 2017 and 2018 to predict the outcome of the vote on election day.

Seat Forecast

Seats

Median number of Democratic seats in our simulations (NOTE this may be more/fewer seats than the strict prediction of seats in the Individual Seat Projections section below, for reasons explained here) :

Dem. Seats: 230 Rep. Seats: 205







Outlined seats are the top 20 “tipping point” districts.

Simulated Seats Over Time

Democrats earn a median of 230 seats in our simulations of the 2018 midterms. This may differ from the strict predictions below because of the larger number of Lean Republican seats than Lean Democratic seats in the current Congress. Effectively we are saying that the below number is an ideal estimate, meant to give you context as to which seats are competitive, but that we expect Democrats to overperform expectations based on the assessment of our error in past predictions.





Individual Seat Projections

Using the average vote share for each district over all of our simulations, we can identify both the districts that have the best chance of flipping parties and the chance that that happens.

Democrats: 221

Republicans: 214

Democrats are likely to pick up 29 seats on November 6, 2018. Republicans are favored to gain 2 seat, for a net gain of 27 seats for the Democratic Party.

Seats Likely to Flip Parties in 2018 District Dem 2016/14 (Margin, %) Partisan Lean (Margin, %) Forecast Dem 2018 (Margin, %) Dem Win Prob. AZ-02 -14 1 10 96 CA-10 -3 1 4 82 CA-25 -6 2 3 73 CA-39 -14 3 1 56 CA-49 -1 1 9 98 CO-06 -8 5 7 91 FL-26 -12 12 4 74 FL-27 -10 14 10 94 IA-01 -8 -2 7 90 IA-03 -14 -4 0 53 IL-06 -18 1 1 54 KS-03 -11 -4 6 86 ME-02 -10 -8 2 64 MI-11 -13 -7 4 77 MN-01 1 -13 -1 43 MN-02 -2 -3 7 92 MN-03 -14 5 6 87 MN-08 1 -13 -3 32 NE-02 -1 -6 1 56 NJ-02 -22 -4 12 98 NJ-07 -11 -3 4 76 NJ-11 -19 -5 3 70 NY-19 -8 -6 3 74 NY-22 -6 -14 1 61 PA-01 -12 -1 2 61 PA-06 -6 5 12 93 PA-07 -10 0 10 96 PA-17 -11 -6 9 94 TX-32 -16 -5 0 53 VA-10 -6 5 9 96 WA-08 -20 0 3 72

Forecasts for every House race on the bottom of the page.





Who’s Vulnerable?

The graph below stacks each House seat on top of each other above the percentage share of the vote I forecast for the Democratic candidate in the district. The gray shaded area represents a 5% margin of error — roughly what we expect given past error in the national generic ballot polls — identifying vulnerable seats that could be won by either Democrats or Republicans.

Tipping Points and the Majority Power Indicator (MPI)

Districts that usually fall in the middle of the pack are “tipping point” districts. They tell us that, in a tied election, these districts most likely land the 218th seat for the winning party. For Democrats, the “tipping point” district is the one that gives them the 24th seat they need to win the election given that they’ve already won 23 other seats (most likely the ones detailed above).

The Majority Power Indicator (MPI) is simply a measure of the increase in the probability that a given party wins the House majority given that they win a given seat. Mathematically, MPI is equal to (1) difference between (A) the number of trials a party wins a given seat and wins the House majority minus the number of trials they win that seat but lose the majority and (B) the number of trials that a party loses a given seat and wins the House majority minus number of trials they lose that seat but lose the majority, (2) all divided by the number of trials/simulations in our forecast model.

Together, the tipping point index and MPI tell us which House districts are most instrumental in producing control of the House majority. More information can be found here.

Tipping Point Districts and the MPI District Tipping Point (% of trials) D. Tipping Point R. Tipping Point MPI (% of trials) WA-08 2.4 2.3 2.5 51 CA-25 2.2 2.2 2.2 51.1 CA-39 2.2 2.2 2.4 33 NJ-07 2.1 2 2.2 54.8 NY-22 2.1 2.1 2.1 38.1 CA-45 2 2 2.3 23.6 ME-02 2 2 2.4 40.3 NE-02 2 1.9 2.2 32.8 NJ-11 2 1.9 2.2 47.9 NY-19 2 2 2 52.3 TX-32 1.9 1.9 2 29.5 CA-10 1.8 1.8 1.5 52 IA-03 1.8 1.8 1.8 27.9 IL-06 1.8 1.8 1.9 28.3 MI-11 1.7 1.7 1.7 53.4 PA-01 1.7 1.7 1.9 35 TX-07 1.7 1.7 1.8 22 CA-48 1.6 1.6 1.7 14.5 VA-07 1.6 1.6 1.6 18.1 KS-03 1.5 1.5 1.8 61.7 NJ-03 1.5 1.6 1.4 17.6 FL-26 1.4 1.4 1.5 41.8 IL-14 1.4 1.4 1.4 6.6 MN-01 1.4 1.4 1.4 13 MN-03 1.4 1.4 1.5 59.1 UT-04 1.4 1.4 1.6 10.3 IA-01 1.3 1.3 1.3 61 MI-08 1.3 1.4 1.2 11 NC-09 1.2 1.3 1.2 3.7 NC-13 1.2 1.2 1.4 2.4 TX-23 1.2 1.1 1.3 -4 CO-06 1.1 1.1 1 64 FL-15 1.1 1.1 1.1 1.2 MN-08 1.1 1.1 1.2 -4.6 NV-03 1.1 1.1 1.2 63.6 AZ-01 1 1 1.1 62.7 MN-02 1 1 1.1 62.6 IL-12 0.9 0.9 0.9 -12.6 NM-02 0.9 0.8 1.1 -17.3 NV-04 0.9 0.9 0.9 61.8 NY-23 0.9 0.8 0.9 -9.7 VA-05 0.9 1 0.9 -9.9 FL-06 0.8 0.8 0.7 -18.3 GA-06 0.8 0.8 0.7 -11.9 NC-02 0.8 0.8 0.7 -18.1 NH-01 0.8 0.8 0.8 62.5 NJ-05 0.8 0.8 0.9 63.8 PA-16 0.8 0.8 1 -15.2 PA-17 0.8 0.8 0.7 62 AK-AL 0.7 0.7 0.7 -19.5

Race Ratings

Each seat is given a discrete race rating based on the following scale for either party: