There have been cheers, as Obama and Romney whooped up supporters one last time, and there have been tears. Now, at last, it is time for the voters to decide. It could be a long night.

Obama at Iowa rally

It is either confidence, or sheer exhaustion. President Obama is spending election day in his home town of Chicago, where, say his aides, he will be playing pick-up basketball with friends and campaign staff, finding time to fit in a few "Get Out The Vote" interviews with television stations in swing states, before a family dinner at home.

Last night, at a freezing cold outdoor rally in Iowa, the emotion clearly showed through, a tear rolling down Obama's face as he surveyed his final crowd at the place where he first started his bid for the presidency four years ago. "It's out of my hands now," he declared. "It's in yours. All of it depends on what you do."

Mitt Romney, on the other hand, is breaking with tradition. He voted early this morning in Belmont, Massachusetts, declaring that he felt "very good" about how things were looking. But there is no time for basketball on his schedule.

He has managed to pack in another hectic dash for last minute votes in Ohio - where he will stop off at Cleveland with running-mate Paul Ryan, and then on to Pennsylvania, a fairly solid Democratic state where the Republicans have been making a surprising push.

It's out of my hands now. It's in yours. All of it depends on what you do. Barack Obama

For if you believe the pollsters and the pundits (a big if), then this election is all about the maths. Almost every poll over the last week has put the candidates neck and neck, and although Obama seems to have an edge in most of the key battleground states, it is all within the margin of error.

That is where the maths comes in. Both candidates need to rack up enough electoral college votes to reach the magic threshold of 270. With so few places still up for grabs, the Republicans made the sudden decision to expand their ambitions into Pennsylvania, with its precious 20 votes.

Romney's backers have spent more than $12m on political ads in the state over the last week, although this could be because the advertising time in the swing states has been completely sold out.

The Republican ground game in the Keystone state has been hugely energised: staffers say they can detect a different mood. "We're taking back the White House because we're going to win Pennsylvania!" Romney told the cheering crowds this week.

But it would be a huge mountain to climb in a state which has around a million more Democratic voters than Republican ones, and where Obama's stronghold of Philadelphia has been trending further towards his party over the last decade.

Obama's chief strategist, David Axelrod, has dismissed any idea of Romney making headway there. This morning he called the play for Pennsylvania "more of a prayer than a plan".

Clinton in Philly

Last night in Philadelphia, former president Bill Clinton was working a 9,500 strong crowd of supporters, his voice breaking at times, but justifying his new title of "Secretary of 'Splaining Stuff", as he told voters how much was at stake.

There were even a few jokes at Romney's expense, and a final appeal for election day. "The America I have fought for since I was a boy is on the line," he declared. "I want you to wake up with light in your eyes, a song in your heart and a spring in your step. Because America is coming back."

If Obama did lose in Pennsylvania, it would be a sign of a massive landslide by Romney, which given how close the election is looking, seems unlikely. Although one poll this week put Obama just three points ahead, not one has put Romney in the lead.

But that has not stopped Mitt Romney from packing in a rally in Pittsburg this afternoon, along with a five-minute election day broadcast featuring some highlights of his campaign, telling voters: "The future is better than the past."

As morning gets underway on this election day, there are reports of long queues at polling stations around the country: hundreds of people queueing long before the polls even opened at 7am, others waiting patiently for several hours.

President Obama dropped by a campaign office in Chicago, giving some encouragement to his "Get Out The Vote" volunteers, rolling up his sleeves to man the phones himself. Who knows what Annie from Wisconsin made of her personal call from the president, as Obama admitted: "She was very nice to me, even though she initially didn't know who I was."

Across the country, though, turnout is on track to reach record levels, a sign, perhaps, of the vast amount of time, effort and resources which both campaigns have been pouring into their drive to drag their supporters to the polls.

And there is no rest for activists. Armed with those lists of voters they have been honing for months, if not years, it will be a relentless dash around those key counties in those key states where every vote could make the difference.

It will be a long day and - if it really is as close as the polls suggest - a very long night.

Felicity Spector writes about US politics for Channel 4 News