A former senior aide to Hillary Clinton has said the Democratic nominee will be the "least popular" winner in the country's history if she triumphs at the polls on Tuesday.

Mo Elleithee, who served as Clinton's official spokesman during her first bid for the White House in 2008 as well chief spokesman for the Democrats until 2015, told STV News that his former boss will face an increasingly polarised political system.

Opinion polls in recent weeks have shown around 55-60% of voters dislike both Clinton and Trump.

Elleithee said: "She will be the least popular president-elect in American history heading into office.

"And will be granted very little of a honeymoon period and a further growingly complicated media system here.

"I worry about the increased polarisation of our political system.

"How she, should she win, moves to try to heal a little bit, to try to unite and try to try to extend to the other side in the transition period is going to be key".

The senior figure said the party did not have Donald Trump on its "radar screen" when planning for the 2016 election last year.

Trump entered the Republican primaries as a long shot with many analysts expecting Jeb Bush to clinch the nomination.

Elleithee told STV News: "When I left the Democratic National Committee in 2015 my job at that point was to spend a year and half preparing for the 2016 election.

"When I say preparing my job was tor really to begin laying the foundation of what our messaging would be against all the potential Republican candidates.

"We did not know who would be in that top tier so we created our own top tier. It was Chris Christie, Jeb Bush, Marco Rubio, Scott Walker and Rand Paul. We were convinced one of those five would be the Republican nominee.

"We had a second tier of folks who we thought could make enough noise in the Republican primary - Ted Cruz and a number of others. Donald Trump was not even on our radar screen".

Polls in recent days have shown a tightening of the race between Clinton and Trump following the FBI's announcement that it has reopened an investigation into a private email server Clinton used to handle official communications as Secretary of State.

The former Clinton adviser believes Trump is "poised to win" Ohio and a "few other of the mid-western states" on the back of the concerns over immigration.

Despite gains by Trump in some areas Elleithee believes the former secretary of state is still heading for victory on Tuesday.

Elleithee is also a close friend of Clinton's running mate Tim Kaine.

He served as a senior adviser and strategist in Kaine's campaigns for the governors and the Senate in Virginia.

He told STV News of the conversation the pair had when she approached Kaine to be her candidate for vice president: "I've heard Tim Kaine say this a lot.

"Kaine often says - I have had this conversation with him personally as well him saying it on the stump - he says you know when Hilary called me and offered me the running mate position she said that 'at the end of our time in office together how I want us to be judged is not on how many bills we got passed, or specific pieces of legislation, as much as will more people feel like they are better off than they were before'."

It is estimated that around 29 million Americans have already cast their vote through the country's system of early voting. That figure is nearly double the number than the previous election in 2012.