Last updated October 20, 2017 01:56:01 PM
Who’s ahead in the race for control of the United States House of Representatives?
|Dem. Chance 45.1%||Rep. Chance 54.9%|
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Odds Over Time
Popular vote percentage based on an average of generic ballot polls:
|Dem. Vote: 42.1%||Rep. Vote: 35.8%||Und./Other: 22.1%|
Two-party projection of election day using generic ballot polls.
|Dem. Vote: 54.2%||Rep. Vote: 45.8%|
Median number of Democratic seats in our simulations (NOTE this may be more/less than the strict prediction of seats in the Seat Projections section below, for reasons explained here):
|Dem. Seats: 215||Rep. Seats: 220|
In my projection of the two-party election day vote share, based on polls of the generic ballot, The Democratic Party is ahead, earning 54.2% of the two-party vote share on average. The margin of error is roughly 4.1% points, meaning the Democrats could earn as little as 50.1% or as much as 58.3% of the vote. Below are the generic congressional ballot polls used to make that projection:
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?
Simulated Seats Over Time
Democrats earn a median of 215 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 are likely to pick up 16 seats on November 6, 2018. Republicans are favored to gain 1 seat, for a net gain of 15 seats for the Democratic Party.
|Seats Likely to Flip Parties in 2018|
|District||Dem 2016/14 (%)||Clinton (%)||Forecast Dem 2018 (%)||Dem Win Prob.|
The graph below stacks each House seat on top of eachother 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.
Each seat is given a vulnerability rating based on the following scale for either party:
- Tossup: win margin <5%
- Lean Dem./Rep.: win margin >5% and <15%
- Likely Dem./Rep.: win margin >15% and <25%
- Safe Dem./Rep.: win margin >25%
Ratings also take into account qualitative factors like candidate characteristics, contest dynamics, fundraising, etc. And, as a general rule, no open seat forecast to change parties will ever be rated “Safe.”
These ratings are available on the model’s companion post at DDHQ
Of course, there is error in our forecasts — measurable error. We can account for that error by simulating the election thousands of times, asking the computer each time to pick random error for national polling that is based on error from past predictions. Then for each “election’s” national error we also generate an error for each congressional district, basing each of those 435 individual errors on what the error is in districts that have performed the same in the past. (So, for example, if a simulated error adds 4% for the Democratic candidate in a district that voted for Romney and Trump by 5%, the error would around 4% for other districts that voted similarly. This is called correlated random error.)
Here’s the distribution of total number of Democratic House seats after engaging in that exercise:
We can take all of those simulations of Democratic seat shares and further ask “what is the chance that Democrats win the election, given the chance that Democrats or Republicans beat expectations?”
Democrats have a 45.1 percent chance of winning a House majority on November 6th, 2018. You can imagine that the rectangle below is a dart board, and if you randomly threw a dart anywhere at the rectangle whichever square you land on corresponds to the party that wins the House majority.
Note: This page will continually be updated, both with new data and new features, from now until Election Day! Come back regularly for a different look at the 2018 midterm race. If you notice something missing let me know.