The HuffPost presidential forecast gives Democratic presidential nominee Hillary Clinton a 98 percent chance of winning the general election on Tuesday. That means we’re pretty darn certain that ― barring some major catastrophe, scandal or nearly every single poll being wrong ― Clinton will be elected.
But that doesn’t necessarily mean Clinton will win in a landslide. It’s still a close race in several states; Clinton could win with as few as 273 electoral votes. Or she could blow the race out and win 341 or more. The high win probability doesn’t choose between those scenarios ― it just means that the model shows Clinton below 270 is very unlikely.
In simple terms, here’s how the model does that: We take all the polls entered into the HuffPost Pollster database in each state and calculate a trendline to estimate what they say in the aggregate. Unlike the Pollster charts, which stop on the current date, the forecast model keeps running to Nov. 8 (although there’s not much difference four days out). And then we bump up the uncertainty in the model to account for the undecided proportions in the polls.
Once we have a probability of each candidate winning in each state, we simulate the election 10 million times: The computer randomly draws a number between 0 and 100 that represents Clinton’s vote share in that state for that election simulation. If the number is below her probability of a win in that state (e.g., the number is 52 and she has a 60 percent probability of winning), that counts as a win for her and she’s awarded that state’s electoral votes. The random numbers are correlated across states since we know state outcomes are correlated.
The electoral votes are tallied once all states are assigned, and the candidate with more than 270 is the winner. The computer repeats that process across all states 10 million times, and the percentage of times Clinton wins more than 270 electoral votes in those 10 million simulations is the probability of her winning the election.
So the 98 percent chance of a Clinton win is not a 98 percent chance that she wins by a certain number of electoral votes ― it’s the percent chance that she wins more than 270 electoral votes. It’s less important that Clinton wins each individual state than it is that she wins enough states in this type of model.
That’s why the HuffPost model is so certain of a Clinton win, even though several states are still close. Ohio, for one, is right on a razor’s edge ― it’s changing directions practically every time the model runs. Clinton is neck-and-neck with Republican presidential nominee Donald Trump as they vie for the state’s 18 electoral votes. Trump needs those to win. Clinton does not.
In fact, Clinton has 302 electoral votes in just the states in which she’s 90 percent or more likely to win. That includes Michigan, Virginia, Pennsylvania, Wisconsin, Colorado, Florida and New Hampshire. That’s because polls in those states have shown consistent trends toward Clinton ― and nothing in the last week has been definitive enough to change those trends.
Some states could continue to close in. New Hampshire is one of those, and Florida is often an electoral conundrum. Most forecast models show a less rosy picture in Florida ― and FiveThirtyEight shows it going for Trump. But that’s based on their poll-weighting structures and adjustments that move a 45.7 to 44.1 Clinton lead to a 48.3 to 48.0 Trump lead. HuffPost Pollster’s estimate of a 2-point lead is within a reasonable range of FiveThirtyEight’s unadjusted numbers. And our model gives Clinton a pretty certain win in Florida because of the consistency of that lead over time.
Without Florida and New Hampshire, Clinton drops to 269 electoral votes ― but North Carolina is also right on the edge of the 90 percent probability mark, and would give Clinton 15 electoral votes to put her over the top. And she’s leading in Nevada right now too. It’s unlikely that she loses all of those states.
The other nightmare scenarios revolve around Clinton losing in states where she has never actually trailed in the HuffPost Pollster aggregates. Those states are Michigan, Wisconsin and Pennsylvania. So of course our model has no reason to think there will be disasters in those states.
Putting all of that together, the model estimates that Clinton is most likely to land at 341 electoral votes ― but it could be as low as 271 or as high as 374. There’s only a 5 percent chance she gets exactly 341, and less than a 1 percent chance she lands at 374 or 271. That means there’s a lot of uncertainty about exactly how much Clinton will win by ― even though there’s a lot of certainty that she will win.