The unexpected announcement Friday that the FBI would review further emails from Democratic candidate Hillary Clinton in connection with its investigation into her use of a private email server is the "October surprise" that could throw the electoral race off course, according to a team of Citi analysts led by Chief Global Political Analyst Tina Fordham. "In our view, these developments do constitute an 'October surprise' that could have a meaningful impact on the race," according to the new research note published on Monday. Fordham's team has consistently warned of the "extraordinarily high" risk of "black swan" events — such as scandals and "information warfare" — occurring during this fraught campaign, hence its reluctance to view a Clinton victory as confidently as many other pundits.

The Citi analysts point to tremors that hit markets in the wake of the bombshell from the FBI that was short on detail but long on impact. The bank states that the news has pushed prediction market odds betting on a Clinton win from 81 percent to 75 percent. Even prior to the announcement, Clinton's lead over Republican candidate Donald Trump had shrunk in the most recent week of RCP polling, from 6.4 percent on Oct. 20 to 5.6 percent on Oct. 27. According to the research, given that it typically takes at least a week for the effect of new developments to be picked up in polls, we may not be able to quantify the full impact of this event until right before next Tuesday's election. The report's authors do not think it is likely the FBI will conclude its probe prior to the elections, adding to and compounding the uncertainty surrounding the issue. Although the Citi team retains its prediction of a 75 percent probability of a Clinton victory, it notes "these developments have added a significant obstacle to the Clinton campaign and are likely to further dent voter confidence."

Former U.S. Secretary of State Hillary Clinton speaks during a news conference at the United Nations in New York, March 10, 2015. Clinton said on Tuesday she did not email any classified material to anyone while at the State Department. Lucas Jackson | Reuters

However, it does not expect her supporters to switch their votes, partly due to a significant amount of early voting having already been carried out and partly due to confidence drawn from an ABC News/Washington Post poll conducted on October 25 – 28 that showed just 7 percent of Clinton voters saying the news makes them less likely to support her. Indeed, the Citi analysts warn "the most concrete impact of the FBI announcement will likely be upon turnout." The research finishes on a sobering note reminding readers that once the election results are counted, the problems may only just be beginning. A Democrat win could see Trump honor his threat to disregard the result as well as the risk of ongoing investigations and the looming specter of future impeachment risk for Clinton — a risk Fordham and her team describe as "non-negligible."