Nate Silver: “As the election season wore on, new polls hewed somewhat more closely to the polling averages. But the change was marginal until the final week or two of the campaign, when they started to track it much more closely. By the eve of the election, new polls came within about 1.7 percentage points of the polling average.”
“Perhaps you could construct some rationale, apart from herding, for why the polls behaved this way. Maybe it became easier to predict who was going to vote and that made methodological differences between polling firms matter less… But there are two dead giveaways that herding happened. One is the unusual shape of the curve. Rather than abiding by a linear progression, it suddenly veers toward zero in the final week or so of the campaign.”
“The other giveaway is… By the end of the campaign, new polls diverged from the polling averages by less than they plausibly could if they were taking random samples and not tinkering with them.”
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