As noted above, I think that with wider central intervals and wider tails we could lower that Biden win probability from 96% to 90% or maybe 80%. But, given what the polls say now, to get it much lower than that you’d need a directional shift, something asymmetrical, whether it comes from the possibility of vote suppression, or turnout, or problems with survey responses, or differential nonresponse not captured by partisanship adjustment, or something else I’m forgetting right now. But I don’t think it would be enough just to say that anything can happen. “Anything can happen” starting with Biden at 54% will lead to a high Biden win probability now matter how you slice it. For example, suppose you start with a Biden forecast at 54% and give a standard error of 3 percentage points, which has gotta be too much—it yields a 95% interval of [0.48, 0.60] for his two-party vote share, and nobody thinks he’s gonna get 48% or 60%. Anyway, start with that and Biden still has a 78% chance of winning (or 75% using the t_3 distribution). To get that probability down below 80%, you’re gonna need to shift the mean estimate, which implies some directional information.
From the blog of Andrew Gelman, one of the authors of the Economist’s model: