On September 8, 2004 Dan Rather oversaw a piece on George W. Bush's National Guard service, less than two months before the presidential election. Through a series of journalistic mistakes the piece centered on forged documents. Not only did Dan Rather lose his job, but voters then discounted the underlying story, which was really a big deal. George W. Bush used the National Guard to avoid going to Vietnam, while his opponent, John Kerry, actually went and fought there. In short, a false story about a true larger story, helped people forget about the true larger story.
On September 8, 2006 (that's right, same date, two years later!) Chris Christie, the US Attorney for New Jersey let it leak he was investigating Bob Mendenez the sitting senator and Democratic candidate. Turns out that Christie was on Attorney General Alberto Gonzales' hit list for the US Attorneys that would be fired for not doing enough to help the Republicans win elections. By letting it leak that Menendez was under investigation, he was saved in the December dismissals, but the stink of the "investigation" follows him to this day. Of course, nothing came of the "investigation" and Christie never pursued anything. In short, announcing investigations without any follow-up has been done before, but it stains the accuser as much more than the accused.
On October 28, 2016 FBI Director James Comey sent a letter to Congress announcing that he was reviewing new emails that may or may not have something to do with Democratic presidential nominee Hillary Clinton's use of private server. After sending the financial and prediction markets into a tailspin, it was slowly revealed over several hours that (1) these emails were on machines owned by the husband of a Clinton aide (2) the emails were probably already checked by the FBI and, if not, were not sent by Clinton, but her aide (3) the Justice Department warned Comey not to send this letter because it would be extremely damaging to the perceived neutrality central to the job of the FBI and Justice Department and not consistent with department policy.
Happening so close to election it is not clear how much this can impact the vote; but, not much is the short answer. (1) It will energize Trump supporters/Clinton haters. This will help with Trump's turnout and down-ballot for the Republicans. Those voters will let this incident reinforce their beliefs of Clinton. (2) It is unlikely to hurt Clinton's supporters support or energy, maybe even help. Like the Dan Rather situation, this will give them further confidence to think of the email scandal as purely political. Clinton made some bad decisions to get to this point, but Comey just confirmed for everyone in Clinton's camp that it was a political hunt. (3) There are very few persuadable voters left and they are unlikely to read past more "he said, she said" over some old scandal involving emails. Seriously, can you imagine how little undecided voters follow the details of politics, if they are undecided after a 500+ day election!
Clinton is up 5-7 points nationally in the polls and 5+ points in 272 Electoral Votes; prediction markets give her an 87 percent to win. Clinton is far enough up, and there are few enough persuadable voters, that she will need to lose support for Trump to win (not just Trump gain support). Almost no one will switch between Clinton and Trump, but some people could switch to non-voters or third party candidates. As I note above, this is unlikely. The markets, down 3 percentage points from noon on Friday, are being a little cautious because they believed/sort of believe that Comey could not be so irrational (this is a technical economics term for making really bad decisions) as to release this letter without any new evidence that Clinton did something wrong. But, every shred of evidence that has been leaked in the last 29 hours (since the letter) seems to confirm that he is that irrational. If he wanted to damage Clinton, but keep his reputation intact, he could have released a longer letter saying that they are reading emails, but they are not from Clinton or some other mitigating point. What he did was a strictly dominated strategy, that did not hurt Clinton more, but, barring new details, will destroy his reputation.
What we have now is a Dan Rather or Chris Christie type scandal. The news coverage is moving past Clinton's wrong-doings to the absurdity of the prosecutor. This is now a Comey/FBI scandal to affect the presidential election. Unless something new emerges, Clinton will likely move right past this (and the new cycle will move onto latest David A. Fahrenthold story on the absurd Trump Foundation). She has an 87 percnet likelihood of winning, knowing what we know right now. So, while there 13 percent he is a hero to President Trump, there is an 87 percent Comey's best job opportunity come January 20, may be as a manager at a Trump Hotel or commentator on Trump TV.