Welcome to the bizarre world of conditional probability
We have a good idea what would happen if Harris wins Pennsylvania. But we're less sure about what things would look like if Trump wins California.
This is very wonky, so we’re going to run it as a bonus Model Talk column for paid subscribers. There should be a free post coming tomorrow or this weekend on a more mainstream topic: whether Kamala Harris can meme her way to victory. -NS
Kamala Harris probably didn’t maximize her chances of winning the Electoral College when she chose Tim Walz as her running mate. We thought Josh Shapiro would be a better choice given his potential to help Harris win Pennsylvania — which has a 34 percent chance of deciding the election. But enough, for now, about the veepstakes. Let’s ask a different question: how much does a single battleground state like Pennsylvania tip us off to the outcome in the rest of the swing states?
Let’s figure it out using our new-ish chart that shows the probability of Donald Trump or Harris winning the election conditional on winning each state. Our forecast thinks Trump has a 46 percent chance of winning the Electoral College, but that jumps to 96 percent when he wins Pennsylvania. Phrased differently: Harris only has a 4 percent chance of winning the election if she loses Pennsylvania.
It’s the same story in the other Blue Wall states: Harris has a less than 10 percent chance of winning the election if she loses any of them. Trump — who is still ahead in most of the Sunbelt swing states though Harris is closing in on him quickly1 — has a few more paths to victory so these states are less important for him. If Harris wins Michigan for example, our model still gives Trump a 17 percent chance of winning the Electoral College.
So these numbers make it pretty clear that Pennsylvania and the Blue Wall states are really important to both candidates’ chances of winning. But where do these probabilities come from?