How 538 is adjusting our election model for Harris versus Trump
On Thursday, Aug. 22, Vice President Kamala Harris officially accepted the Democratic Party's nomination to be its 2024 candidate for president. The following day, we turned on our election forecast for the race between Harris and former President Donald Trump.
But before you dive into the numbers, I wanted to explain how we've changed the model that powers that forecast. These changes address issues that became apparent while running the model for President Joe Biden and Trump in June and July. We have also adjusted our model to make sure it could properly handle cases when major third-party candidates, such as Robert F. Kennedy Jr., drop out of the race.
At a high level, the version of the model we published before Biden dropped out of the race allowed for a dynamic rather than static, explicit weighting on the polls. When we launched the forecast in June, this was not an apparent issue; the model was generating estimates that were only slightly closer to our "fundamentals" indicators about the election — such as economic growth and whether an incumbent president is on the ballot — than to the polls, which had a lot of uncertainty in early June.
In July, however, the two sets of indicators started to diverge significantly. As the polls moved away from the fundamentals, the model did not react as expected to the new data and therefore generated estimates that deviated from the polls. (This divergence was possible because the old model did not explicitly dictate how much weight to put on polls versus fundamentals. More details on that below.)
