Investors fear a Brexit-style shock tomorrow morning...
It has been a presidential campaign of peaks and troughs for stock market values - culminating in a relief rally on Monday following confirmation from the FBI that Hillary Clinton would not face charges over her past use of a private email server.
The Democrat is seen as the continuity candidate offering greater clarity on issues such as trade while, on the other hand, her Republican rival Donald Trump's stance on foreign policy, trade an immigration is viewed as more uncertain.
A narrowing of the polls amid the FBI probe triggered the longest losing streak for the S&P 500 since 1980.
It gained 2% on Monday in the first session since Mrs Clinton was officially cleared while other world markets including the FTSE 100 also clawed back some lost ground.
Values were largely flat on Tuesday as polls opened with the S&P 500 0.3% lower. The pound was slightly down against the dollar, just below $1.24.
Polls suggest Mrs Clinton has the edge again but many analysts believe the race is too close to call and traders have hedged their positions. Some pointed to investors getting their fingers burned over the shock vote for the UK to leave the EU.
Mizuho Bank said in a report: "Markets are moving toward pricing in a Clinton victory, but not fully, given the painful lesson from Brexit.
"We expect market volatility to increase somewhat tomorrow as US exit polls and elections results start streaming in."
Neil Wilson, markets analyst at ETX Capital, said he was expecting one of the most turbulent trading sessions since the referendum on Brexit.
"Whatever the result, it's going to be a sleepless night for our traders as the results come in throughout the early hours of Wednesday morning," he said.
"We'll be fully staffed overnight to cope with the increased trading and added volatility.
"Market action could hinge on one or two moments - for example the Florida result, key for Trump's chances for the presidency, is vitally important."
He added: "If key states start to lean towards Trump, we may see sizeable risk-off selling as investors flee to havens like the Swiss franc, Japanese yen and gold."
Chief markets analyst at Think Markets UK, Naeem Aslam, warned a Trump victory could push the S&P 500 6% lower on Wednesday.
Newstalk will have live results coverage of the election starting at 10pm tonight on air with a special election show on air, as well as live updates and analysis all night on Newstalk.com.
You can also follow along on Twitter @NewstalkFM.