The world's most extravagant presidential race reaches its climax

Rupert Cornwell introduces our pull-out companion for insomniac UK viewers

At last – and most would say not before time – the only poll that matters is at hand. Today Americans will elect either Barack Obama or Mitt Romney as their next president. Which man an estimated 130 million or more voters will choose, is as uncertain now as it has been almost all year.

The campaign has been a record-breaker in many ways, not all of them desirable. An unprecedented $2bn (£1.2bn) will have been spent on the presidential vote alone, more than $6bn if Senate and House elections are included. More ads will have flooded the airwaves than ever before. Yet fewer states were seriously contested than in any recent election, leaving much of the country unvisited by a candidate and in spirit almost disenfranchised.

Some constants however hold true. History shows that incumbents are rarely defeated. With the exception of 1976 and Gerald Ford (who was never elected in the first place), it has happened only twice in three-quarters of a century. On the other hand however, the ever-growing polarisation of US politics means that close elections are now the norm.

A Roosevelt or Reagan-style landslide is inconceivable in today's divided America. George W Bush won a couple of squeakers, while Mr Obama's 2008 margin of 365-173 in the Electoral College may have been a high water mark for any candidate.

This time, he will do well to garner 300 electoral votes. As for the challenger, barring an undetected eleventh-hour break in Mr Romney's favour, a win might be even narrower, with few even among his own supporters giving him more than 280 or so, barely above the 270 needed to win. A 269-269 tie is not totally out of the question; nor is a split verdict, where one candidate (more probably Mr Romney right now, according to analysts) wins the popular vote but loses the Electoral College. If there are recounts and legal disputes, the winner may not be known for days, perhaps longer.

For all the ups and downs, the contest has ended up more or less where it started: too close to call. From early in the Republican primary season, Team Obama never doubted two things: that 2012 would be a much more difficult and very different campaign from 2008; and that Romney would be the opponent in November.

The result was a grinding and mostly negative campaign, concentrated on a handful of swing states, in which the Obama campaign quickly moved to define Mr Romney as an elitist corporate raider, in thrall to his party's right wing. The challenger was slow to fight back, and did not help his cause with a series of gaffes, most notably his secretly recorded riff at a closed door Florida fundraiser about 47 per cent of Americans who considered themselves "victims".

Even so the race was neck and neck, except for the single month of September, between the Democratic convention and the first presidential debate, when Mr Obama opened up a lead beyond the statistical margin of error and even many Republicans were starting to give up on their candidate.

That fleeting advantage evaporated on 3 October and the first debate in Denver, where Mr Romney was as commanding as the President was listless. Momentum passed to the challenger, wooing independent voters with a more moderate stance. Not only did the former Massachusetts governor pull into a narrow lead in national polls, he also chipped away at Mr Obama's advantage in the 10 or so battleground states that the latter carried in 2008, in some of them overtaking him.

There matters stood until Hurricane Sandy stopped the campaign in its tracks for three days. Suddenly, the strident candidate Obama became Commander-in-Chief Obama, monitoring the crisis from the White House Situation Room, even basking in praise from Republican Chris Christie, governor of New Jersey, the state worst hit by Sandy. Mr Romney perforce was reduced to onlooker. Early voting notwithstanding, if Sandy helped anyone, it was Mr Obama.

But with few undecideds, the result may hinge on the "ground game" – the ability of each side to get supporters to the polls. Republicans appear to have the energy, the Obama campaign the better organisation. Which will prevail, no one can be sure.