Why did Kamala Harris’ chances of winning just jump?
HeyYour stats The US presidential election model will be updated six more times before the votes are counted. There are few opportunities for the candidates to move the dial in an election that has been stubbornly close since Kamala Harris became the Democratic nominee. Today’s update will delight her supporters: The vice president’s probability of winning has risen by six percentage points, making the race very hot.
There are three reasons. The first is the volume of new polls – 65 polls were added to our forecast today – which gives the model more confidence about small changes. The other reason is that there is little time left before the elections. So far, our model has been a forecast, with candidates weeks or months away from making gains. Many pollsters are now publishing their final polls of the cycle, so the forecast will soon become “expected now.”
The third is that the race is remarkably close, which means that even small changes in expected vote shares can lead to big shifts in the odds of winning. Yesterday’s most influential opinion polls were concentrated in four states: Michigan, North Carolina, Pennsylvania, and Wisconsin. In those states, Ms. Harris’s expected vote share rose by an average of 0.4 percentage points (see chart) — a small step that was nonetheless enough to increase her chance of winning by an average of six percentage points in the four states.
On the surface, the new polls didn’t look unusually good for Harris. Most showed results that were close to a tie. However, the companies that released polls yesterday — especially AtlasIntel, Quantus and Trafalgar — tended to give Donald Trump better numbers this year than other pollsters who polled the same races at similar times. Our model changes all survey results to counteract such biases. On average, these adjustments pushed the voting margins in yesterday’s swing polls in swing states about half a percentage point in Ms. Harris’s direction.
Moreover, in recent days, the model has been trending toward Trump, and Ms. Harris’s average projected vote share (excluding third parties) has fallen below 50% in every swing state besides Michigan. As a result, new polls showing a tied race (such as the one in Pennsylvania did on average after our adjustments) or even a small lead for Trump (as the one in North Carolina did) still represent an improvement for Ms. Harris, compared to With the form. Her forecast was relatively bleak yesterday.
New opinion polls also appeared in Arizona and Georgia yesterday, with results widely spread, ranging from Trump’s lead by eight points to Harris’ lead by one point. However, after our adjustments, these new polls averaged very close to the model’s previous prediction of a two-point lead for Trump in both states. As a result, the forecasts for Arizona and Georgia are unchanged.
Harris’ small gains bring her back into parity in Nevada, Pennsylvania and Wisconsin and make her a narrow favorite in Michigan, while Trump maintains a small but clear advantage in Arizona, Georgia and North Carolina. Each candidate won half of our model’s simulations in its most recent round. On average, they would each receive 269 electoral votes, which would leave the House of Representatives breaking a tie, likely in Trump’s favor. However, the model gives less than a 1% chance of an actual tie in the Electoral College, which would require Ms. Harris to win Michigan, Pennsylvania and Wisconsin while losing Nebraska’s 2nd Congressional District.
The direction or magnitude of survey errors cannot be predicted. But if history is any guide, surveys are likely to underestimate a candidate by a margin dwarfed by the small daily shifts in our model’s average estimates. Any such error would likely result in a decisive victory for any candidate who takes advantage of it. Despite tight polling, our forecast gives a two-in-five chance that the winning candidate will receive more electoral votes than Joe Biden did in 2020 or Trump did in 2016.
The other main source of uncertainty in our model, apart from polling errors, is the time remaining until the election. The forecast works by estimating candidates’ current positions with available data, then simulating the movement that could occur each day until November 5. With only six remaining, there was little action left to do.
The effect on our predicted probabilities is counterintuitive. There are few opportunities for significant changes in public opinion, which means that opinion polls published now carry more weight. As a result, forecast probabilities may change more from day to day than they did earlier in the cycle. The slight movement in Ms. Harris’ favor today is harder to reverse in the next six days than it was a month ago.
The polls in today’s forecast update were based mostly on interviews conducted a few days ago, so it’s difficult to judge what, if anything, caused an uptick in Ms. Harris’ standing. Some of the polls now being released were taken after Trump’s Madison Square Garden rally on October 27 — which is now widely viewed as a misstep for his campaign — but we’re unlikely to have a clear idea of that until after the election. Whether this event moved many voters remains to be seen. It looks as if the last six days of the election campaign will proceed in a similar way to the past three months: there is a lot to talk about, but no decisive leader. ■