For confirmation, we need to look no further than 2016, when polls misfired in key states such as Michigan, Wisconsin and Pennsylvania. Narrow wins there propelled Trump to an unexpected Electoral College victory, despite his losing the national popular vote.
State polls were important to the forecast models of, among others, HuffPost, which declared just before the election that Clinton’s probability of victory was 98.2 percent, that she would “fairly easily hold onto Michigan, Wisconsin and Pennsylvania,” and that Trump had “essentially no path to an Electoral College victory.”
In a brave and revealing mea culpa two days after the election, HuffPost’s polling director, Natalie Jackson, wrote: “The problem was that I placed way too much faith in polls. I assumed they would be right. … I kept looking at the consistency of the polls. They wavered in the exact margins, sure, but always showed Clinton winning in the key states that she needed to win. I saw no reason to question that the polls would be accurate overall. So I defended and stood by the numbers — as anyone who trusts their work does. That’s left me eating some crow.”
Read the whole thing, which is an excerpt from Campbell’s new book, Lost in a Gallup: Polling Failure in U.S. Presidential Elections.