In one of the closest presidential elections in American history, anything and everything could have been decisive. But we’ll probably never know what really was.
The letter about Hillary Clinton’s emails that the F.B.I. director James Comey sent to Congress in October 2016 is one of these potential factors. Mrs. Clinton herself told Fareed Zakaria on CNN last fall that she “would have won but for Jim Comey’s letter on October 28th.”
But as the Department of Justice concluded its report into Mr. Comey’s handling of the investigation into Mrs. Clinton’s email server on Thursday, it is still unclear whether this letter cost her the presidency.
The Case for Yes
Mr. Comey’s letter came about one week after the third presidential debate and less than two weeks before Election Day. At that time, most polling averages showed Mrs. Clinton ahead by around six percentage points in national polls. A week later, her lead had declined to three points.
The three-point shift against her is often attributed to Mr. Comey’s letter — three points being more than enough to cover Donald J. Trump’s narrow margins in Michigan, Pennsylvania and Wisconsin. He won each state by less than 0.8 percentage points.
Late-deciding voters broke overwhelmingly for Mr. Trump, the exit polls showed, and the Comey letter and its disclosure of new information in the email investigation was a significant part of the news coverage over the last week of the election.
The Case for Not Being So Sure
It’s a plausible case, but there’s a problem: Mrs. Clinton’s support was probably already in decline before the Comey letter.
This decline makes it harder to claim definitively that Mr. Comey’s letter was responsible for any subsequent decline. The trends leading to Mrs. Clinton’s defeat — in particular, Mr. Trump’s consolidation of hesitant Republican support — may have already been underway.
The ABC/Washington Post tracking poll, for instance, showed Mrs. Clinton’s lead dwindling to just two points in its last poll before the letter, down from a double-digit lead after the third debate. One of our own polls — a collaboration with Siena College — showed Mr. Trump leading in Florida in the days before the letter. No live interview poll had shown Mr. Trump with such a large lead in Florida since early July.
Some polling analysis has treated these two surveys as evidence of a Comey effect, because they showed Mrs. Clinton’s lead slipping, and they were released after