A former CIA operative and self-described member of the "Mormon Mafia" is on track to steal an unlikely election victory in Utah that could prove a devastating "October surprise" for both Clinton and Trump.
Independent candidate Evan McMullin only joined the race in August out of a disillusionment with the extreme rhetoric of Donald Trump.
The Utah-born former counter-terrorism adviser and investment banker has described himself as a "nobody" whose chances of winning have been likened to a "moon shot".
However the understated operator has proved a national sensation in the dying days of one of the most bitter and controversial campaigns in history.
The latest Utah polls show him within the margin of error against Donald Trump in what has been a Republican state for the last 50 years.
If McMullin does manage to win the state and its six electoral college votes, it could prove a devastating upset for the Trump campaign who have come within one percentage point nationally of Hillary Clinton following revelations more emails have been seized from the laptop of disgraced Democrat Anthony Weiner this week.
On Monday, the unmarried McMullin came under attack from white nationalist Trump supporter William Johnson who branded him a "closet homosexual" and highlighted the fact his mother married another woman after divorcing his father.
McMullin hit back hard, saying it's consistent with "Trump's bigoted, deceitful campaign and vision for America. Utahns won't be fooled."
"I'm a nobody they say, I'm proud to be a nobody," he told Fox News on Monday.
"We're not trying to win 270 votes," he said about his strategy of appealing to moderate Republicans in Utah which is around 60 per cent Mormon.
"What we're trying to do is earn enough electoral votes to block Hillary Clinton and Donald Trump if the race between both of them is so close that we are able to do that by winning one or two states."
'YOU WERE HARRASSING WOMEN, I WAS FIGHTING TERRORISTS'
The Provo-born McMullin who has never run for office has been described as a "little-known policy wonk turned political sensation" who has a real shot at winning Utah's six electoral college votes. His running mate is Republican digital strategist Mindy Finn.
University of Birmingham US politics professor Scott Lucas said McMullin's success has taken many by surprise, but there is "certainly a possibility" he could win.
He said many of Utah's Mormons have been reviled by Trump's attitude to women and anti-immigrant rhetoric which is at odds with their religious values.
"Utah should be a solid Republican state hands down, but a lot of Mormons were very unsettled by Trump. They took offence at some of the remarks about women ... [and were] troubled by his remarks on immigration ... Any type of hostility towards a group puts them on edge," he said.
The former national security adviser and republican policy director has played up his differences to Trump and campaigned on traditional right-wing values of small government, low taxes and pro-life policies.
Republican campaign adviser Rick Wilson told the LA Times: "We wanted a place for conservatives to say, 'I cast a vote in 2016 that I didn't feel I had to take a shower in bleach after.'"
While the decade McMullin spent undercover in the security service ensures much of his past remains classified or unknown, former colleagues have described him as trustworthy and hardworking.
"Yes you've never heard of me because while you were harassing women at beauty pageants, I was fighting terrorists abroad," he tweeted at Trump this week.
He has come under attack from Trump supporters who have claimed he is a plant or a security risk given his covert background.
Fox host Lou Dobbs said he was a "Mormon mafia tool" leading him to claim the spoof label for himself online.
Prof Lucas said McMullin's late surge exposes the huge chasm in the Republican Party that has been exploited by Trump.
"What you're seeing through McMullin is not the idea that the Republican establishment are getting it back together.
"It's a manifestation Republicans aren't dealing with the divides," he said noting the difference between the more moderate wing and "red meat" Republicans.
"Utah is only six electoral votes, but if the trend of the election is going the way it looks over the last week, every electoral vote is going to matter. Certainly losing six of those votes will dent Trump's hopes."