What’s gonna happen between now and November 5?

The above graph is from our 1993 paper, I guess I should update it sometime . . . anyway, the point is that public opinion used to move a lot during presidential election campaigns. We’re now about 70 days before the election, and even in that period you can see some real movement. For example, in 1988 the polls moved by about 3 percentage points during those last 70 days, and in 1976 they moved about 9 points. At the time of this writing, Harris is polling at about 51.5% of the two-party vote; a shift of even 3 points would make a huge difference.

We talked about this last month (also here): a key difference between different election forecasts is what possibilities they were including in their models for (a) changes in public opinion between now and election day, and (b) systematic polling error.

Can anything be said about the possible directions of (a) and (b)? I’m not sure. The starting point is the so-called fundamentals-based prediction based on the economy, incumbency, and presidential popularity. I think the fundamentals-based prediction is pretty close to where the polls currently are, so that won’t do much for us. I don’t think polls or public opinion are a random walk; it just happens that they’re around where we’d expect them to be based on external conditions. Which is not a huge surprise, given that in our modern age of political polarization, most voters have already “come home” to their parties very early during the campaign.

The next thing is momentum. I don’t really buy “momentum” as a general explanation, but there are specific pieces of information such as fundraising, crowd sizes, etc. Voter enthusiasm can go both ways. As a candidate, you want enthusiastic voters, no question. At the same time, enthusiasm can also show up as a greater rate of response (see this research article, or this shorter version). So Harris supporters being revved up and Trump supporters being demoralized could result in differential nonresponse making the polls look better for the Democrats than they really are. On the other hand, pollsters should be correcting for that! It’s easy enough; you just ask respondents their party ID and who they voted for in the previous election, and adjust accordingly. OK, it’s not trivial, as you still have to decide what to adjust for, but at least it should give more stable polls.

What about systematic polling error? In 2016 and 2020, the Democrats did better in the polls than in the election, and this varied by state. Variation by state is unavoidable; will there still be an average polling error? I don’t know. On one hand, pollsters know what happened in the previous two elections and are trying to do better; on the other hand, they knew about this in 2020 and that didn’t stop them from being off by two percentage points.

Finally, there’s whatever other information is out there that hadn’t made its way into the polls. For example, if the Robert Kennedy Jr. endorsement will affect turnout by bringing anti-vax and conspiracy-minded people to the polls for Trump, or by turning non-conspiracy-minded swing voters away from him, that might not be reflected in current polls but could lead to a shift in the next two months. And then there’s the consistent unpopularity of Trump and Vance: this should already be affecting the trial-heat polls but perhaps could have an impact on undecided voters? There’s also the idea that the Democrats are running a more competent campaign, which could help them as the election grows closer. But I’m not really sure what to make of such arguments, so for now I’ll have to go with our current forecast giving Harris a 60% chance of winning. (For reasons discussed here, I don’t think it makes sense to specify the probability any more precisely than that.)

Again, check out the graphs above to see how much things have changed. Right now, it seems reasonable to expect the polls to stay pretty much the same between now and election day. If they were to move by 3% in either direction, yeah, that’s certainly possible, but it would be considered a big swing. Until recently, though, the saying was that the campaign didn’t even start until Labor Day. And back when we wrote that paper, we found that the fundamentals-based forecasts performed much better than polls-based forecasts. State polls are fine, but this whole obsessing-over-the-state-polls is getting out of control. They’re part of a balanced forecasting approach (see also here) which allows for many sources of uncertainty.