From the Guardian, Dead-heat poll results are astonishing – and improbable, these experts say:
Josh Clinton, a politics professor at Vanderbilt University, and John Lapinski, the network’s director of elections, pondered whether the tied race reflected not the sentiments of the voters, but rather risk-averse decision-making by pollsters. Some, they suggested, may be wary of findings indicating unusually large leads for one candidate and introduce corrective weighting.
Of the last 321 polls in the battlegrounds, 124 - nearly 40% - showed margins of a single point or less, the pair wrote. Pennsylvania was the most “troubling” case, with 20 out of 59 polls showing an exact tie, while another 26 showed margins of less than 1%.
This indicated “not just an astonishingly tight race, but also an improbably tight race”, according to Clinton and Lapinski.
Large numbers of surveys would be expected to show a wider variety of opinion, even in a close election, due to the randomness inherent in polling. The absence of such variation suggests that either pollsters are adjusting “weird” margins of 5% or more, Clinton and Lapinski argued – or the following second possibility, which they deemed more likely.
“Some of the tools pollsters are using in 2024 to address the polling problems of 2020, such as weighting by partisanship, past vote or other factors, may be flattening out the differences and reducing the variation in reported poll results,” they write.
Either explanation “raises the possibility that the results of the election could be unexpectedly different than the razor-close narrative the cluster of state polls and the polling averages suggest”, they added.
Polling firms always adjust their results in an attempt to address any problems they suspect with the data. If they're over-adjusting, they could be skewing the results they're reporting.