Here's how much of a hit Biden's presidential chances would take from a Comey-like October surprise

• About a week before the 2016 election, former FBI Director James Comey sent a letter to Congress alerting them to "the existence of emails that appear to be pertinent to the investigation" of Hillary Clinton's private email server.
• According to FiveThirtyEight's Nate Silver, the Comey letter cost Clinton 4 percentage points of support in key states and nearly 2 points nationally.
• If a similar event were to happen to Democratic nominee Joe Biden, he is still projected to lead President Donald Trump nationally and in most of the tipping point states.
• Due to much higher rates of early and mail-in voting in 2020, the chances of an event upending Biden's campaign decrease as it nears Election Day.

Former FBI Director James Comey sent a letter to Congress on October 28, 2016 revealing "the existence of emails that appear to be pertinent to the investigation" of Hillary Clinton's private email server.

Clinton's popular vote forecast was narrowing as the election drew closer, but Comey's letter may have affected the voting margin by up to 4 percentage points in the most important states of the race and close to 2 percentage points nationally, according to a report by Nate Silver, founder of FiveThirtyEight, a poll analysis website.

If a similar event were to involve Democratic presidential nominee Joe Biden, his projected positive voting margins would shrink, but not enough to ruin his chances in key tipping point states, FiveThirtyEight projected.

The seven states with the highest chance of nudging a candidate over the 270 electoral vote mark are Pennsylvania, Florida, Wisconsin, Michigan, Arizona, North Carolina, and Minnesota, FiveThirtyEight forecasting shows. In 2016, President Donald Trump won all of these states except Minnesota, which Clinton claimed by about 2 percentage points.

An event with the same impact as Comey's letter would reduce Biden's projected lead in Pennsylvania, a state the former vice president absolutely needs to flip, from 5.6 to 1.6 percentage points, according to projected voting margins from FiveThirtyEight. His estimated share of the national popular vote would then lead Trump by just 1.8 percentage points.

With 29 electoral votes, Florida has the most electoral votes of any state that swung from blue to red in 2016. If a surprise development rocked Biden's chances before Nov. 3, Trump would become the new projected favorite in Florida with a lead just under 2 percentage points. According to simulations by election forecasting company, Decision Desk HQ, Trump wins a second term only 10% of the time without landing Florida.

Any similar last minute developments would also diminish Biden's lead in Arizona and North Carolina, FiveThirtyEight projects. Both states went to Trump in 2016 for a combined 25 electoral votes. Neither Biden nor Trump are completely reliant on these states to win, but their electoral votes allow for an easier path to the White House.

As of Oct. 8, over 6 million people have already cast their votes through mail-in and early voting in an election expected to receive the most mailed votes than any other in US history. At this point in 2016, about a month away from the election, only 75,000 mail-in votes had been cast. According to voting rights experts, voting by mail is expected to make up 50% to 70% of the total vote in November.

It's impossible to predict if a highly scrutinized incident could occur right before the election and ruin Biden's chances. But, the closer a Comey-like event occurs to Election Day, the lower the likelihood of it upending the Biden campaign as a much greater proportion of votes will already be placed