Simulating the 2020 Democratic primaries with instant-runoff voting
Here I aim to use Morning Consult’s most recent 2020 Democratic primary polling data to predict the results of the Democratic primary if a) it were held right now, b) it were held nationally similarly to a general presidential election, c) the MC results are perfectly accurate, and d) only the 5 candidates for which MC includes data on voters’ 2nd choices (Biden, Sanders, Warren, Harris, Buttigieg) were running. Oh yeah, and if the race was held with instant-runoff voting — so we keep going until someone has >50% support (at least among the remaining candidates). In other words, we keep going and eliminating the lowest-performing candidate until a single candidate has either 50% of the total vote or (more likely) 50% of the vote of all candidates we have not yet eliminated.
Currently Biden is in the lead with 37% support, followed by Sanders (19%), Warren (11%) and Harris and Buttigieg (7% each). Who do Harris and Buttigieg supporters pick as their second choices?
For Harris: 26% Biden, 21% Warren, 12% Sanders (and 41% other).
For Buttigieg: 25% Biden, 21% Warren, and 18% Harris (and 36% other).
So let’s first give Biden both 26% of Harris’ 7% support and 25% of Buttigieg’s 7% support — i.e. (26+25=51%)*(7%) = 3.57%. We’ll skip the support Harris would get from Buttigieg since she (like him) will be eliminated in the first round. So then Warren gets 21% of Buttigieg’s 7%, or 1.47%. Then Warren also gets 21% of Harris’ 7%, or 1.47%, and Sanders gets 12% of Harris’ 7%, or 0.84%. So:
Biden = 37% + 3.57% = 40.57%
Sanders = 19% + 0.84% = 19.84%
Warren = 11% + 1.47% + 1.47% = 13.94%
OK, so who do Warren’s supporters prefer as their 2nd choice? Answers: 24% Harris, 22% Sanders, 16% Biden (and 38% other). So now that Warren has 13.94%, we then give Sanders 22% of that (3.07%), and give Biden 16% of it (2.23%). Since Harris is already out of the race, the rest of Warren’s support just vanishes as she is eliminated, and then we get:
Biden = 40.57% + 2.23% = 42.80%
Sanders = 19.84% + 3.07% = 22.91%
So then Biden wins by a little less than 20 points, because he has over 50% of support out of the 2 remaining candidates. Specifically, he ends up with about 65% of the vote in the final round against Sanders based on the results shown just above.
Unfortunately, almost a third of votes here (about 33%) do not factor into the final result at all. Why? First of all, we are only looking at the top 5 candidates, who combined get 81% of the vote, so that means 19% of votes will not be counted here at all. The remaining 14% of the vote that is “missing” is eliminated because it would go to candidates who were already eliminated because they were finishing too low.
