Less than a week from the deal going down, polls are showing an insanely close race between Donald Trump and Kamala Harris, both nationally and in the seven battleground states. Lately, it has been hard to find polls showing much of anything else. And that has led to suspicions that, as often happens in the home stretch, pollsters are “herding,” i.e., making their numbers as close as possible to those of other pollsters, as Nate Silver put it:

There are too many polls in the swing states that show the race exactly Harris 1, TIE, Trump 1. Should be more variance than that. Everyone's herding (or everyone but NYT/Siena).

Silver has been focused on herding for some time. When he was still running FiveThirtyEight he offered an explanation for why some pollsters herd:

One further complication is “herding,” or the tendency for polls to produce very similar results to other polls, especially toward the end of a campaign. A methodologically inferior pollster may be posting superficially good results by manipulating its polls to match those of the stronger polling firms. If left to its own devices — without stronger polls to guide it — it might not do so well. When we looked at Senate polls from 2006 to 2013, we found that methodologically poor pollsters improve their accuracy by roughly 2 points when there are also strong polls in the field.

To put it another way, nobody wants the embarrassment of publishing a final preelection poll that turns out to be a complete outlier. Perhaps that’s why some of the worst outliers are actually produced by “stronger polling firms” that don’t worry about their reputation for accuracy, including the New York Times–Siena outfit that Silver singles out as honest with its data. Times–Siena had Joe Biden up by nine points among likely........

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