The United States has an electoral system that is not organised, designed or funded to cope with "anywhere near a 100% turnout" as an estimated 130 million Americans head to the polls, according to the director of a leading independent electoral reform group.
Doug Chapin, director of The Pew Charitable Trust's Electionline.org, said voter turnout will "dwarf" all other problems in this year's presidential election as an estimated 70% of registered voters try to cast their ballots.
"The challenge is we will get closer to 100% turnout on election day this year than ever before," he said.
Mr Chapin said the US typically sees a turnout in the "high 50s to 70% range" but added: "You now see some states forecasting, 80%, 85%, even 90% turnout of their registered voters on election day.
"And this flood of new voters is going to challenge the system in a way that it really never has been before."
He said if there was a problem at the front of the line at a polling station, this was an inconvenience if there were 10 people in the queue.
But he went on: "If there are 100 people in line it is a problem; if there are a 1,000 people in line, it's a crisis.
"Given the number of folks that we have coming out to vote this year, any problem that occurs at the point of voting has the potential to be a real challenge on election day."
He said many states were "overwhelmed and in many ways overrun" by the number of voters during the primary season, but were "fully prepared, or what they think is fully prepared, for record turnout across the country" on Tuesday.
"It is an article of faith and job mission of every election official in the United States that every eligible American who wishes to do so should have the right to cast a ballot and have that ballot count," he said.
"The truth however, is that, given how decentralised our system is and the disputes over our system, we do not necessarily have a system that is organised, designed or funded to handle anywhere near a 100% turnout."
He said the voting system was "startlingly" decentralised.
"It is a myth that there is a United States' election system," he said.
"We have at least 50 separate state systems; in actuality, probably closer to thousands of state and local election systems.
"I would be very surprised if we don't hear more of, 'We need to centralise elections more' after this election."
He said the problems caused by voter turnout were already being seen in reports of early problems related to long queues, scattered reports of machine problems, and a "system characterised by overwhelming demand".
Asked if the US could see another problem such as the one in Florida which dogged the 2008 election, Mr Chapin said: "I can state unequivocally, maybe we will."
He said three states would be worth watching closely for any problems.
Florida, the home of the "hanging chads" and spoiled ballots of 2000, has been a "symbol of election reform" since and has seen "as much change, if not more, than any other state in the country".
Ohio, a "plumb political catch" is usually very closely fought and "no dispute in this country has taken place since 2000 without taking place in some meaningful way (here)," he added.
And Colorado, also a key battleground state this year, is "as unsettled in its election administration as any state in the country right now", he said.
It was one of the last in the country to complete a required upgrade to its state-wide voter rolls, its chief state election official is a candidate for the US Congress, and its state election director recently resigned.
Mr Chapin, who also wrote a report subtitled "What if we had an election and everyone came?", said: "Many people out there are predicting problems at the polls. We don't yet know that that is warranted.
"If we have a problem... it will be because of something completely unexpected, not because of a lack of preparation."
He said voters had shown increased interest this year, not just in the candidates but also in the mechanics of casting their votes.
"So while we won't know until polls close on election day whether or not we have avoided the problems of the past, there are signs for optimism."
In the UK, the 2005 general election saw a national turnout of 61.36%.
Last week, MPs were told that Britain should learn from the expected high turnout in the presidential election.
Commons leader Harriet Harman said a lack of voter registration and low turnout was something that had to be tackled in Britain, particularly among people living in inner cities.
"It looks set to be an election with very high turnout from people who previously have not necessarily voted, people who have registered to vote and then have gone out to vote," she said in the House of Commons.
"One of the things that all of us should be preoccupied to tackle is lack of registration, particularly in inner city areas and among poorer people and low voting turnout.
"If there is something we can learn from the American elections about more people voting and more young people voting then that is something we should look to."