The AAPOR report on 2016 “polling failure”

[Pericles]

Their conclusion on Comey is wrong. They fail to note that while Clinton was trending downward slightly before the Comey letter, her numbers crashed after the letter and she lost over half her lead in less than a week. On October 28, her lead was 4.6%, just 5 days later it was 1.7%. On October 23 her lead was 5.6%, so Clinton's fall accelerated after the letter. Her lead of 3.2% on Election Day(RCP average) was 1.4% smaller than before the Comey letter. While some of that support was recovered, her lead was still significantly smaller on Election Day than before the letter was released. It's also possible that the slight national polling error was caused by Comey exonerating her(which he would never have needed to do if he hadn't sent out the letter in the first place!) and bringing 'Clinton emails FBI' back into the headlines and the minds of voters right before they went to the ballot. I'd say Comey cost Clinton Michigan, Wisconsin, Pennsylvania and Florida. Nate Silver agrees Comey cost her the election, and she was quoting him when she said that if the election was on October 27 she would be President. And given the closeness of the election, even if Comey only shaved 1% of her lead and had a small effect, that would have been enough to flip the result and I'd say that the effect was higher.