The largest, the longest, the costliest and the cruellest exercise in democracy on the planet is approaching its climax. Thirty-six days from today (barring a repeat of the Florida deadheat a dozen years ago) a new American president will have been elected – in the event of a victory by Mitt Romney, the 45th in a line stretching back to 1789 and George Washington.

The winner will be the last man standing after a contest that formally began with Iowa's caucuses last January, and continued through a four-month primary season and the late summer party nominating conventions. Now come four presidential and vice-presidential debates, capped by a final draining sprint to the finishing line on 6 November. In reality, though, the process has been under way almost from the instant Barack Obama was sworn into office on the freezing Washington morning of 20 January 2009, promising a new beginning for his country in the midst of its worst economic and financial crisis since the Great Depression.

The final match-up is the one that all along has seemed likely, between the Democratic incumbent seeking his second permitted term, and a Republican challenger who if truth be told never stopped campaigning for the White House even after he had lost his party's 2008 nomination to John McCain. By the time it's all over, some $3bn may have been spent on the presidential election alone, in money raised by the candidates, their respective parties and outside groups (not least the infamous Super PACs, empowered by a Supreme Court ruling that enables super rich donors to contribute as much as they like).

Throw in the similar sum likely to be spent on the down-the-card contests, for all 435 seats in the House of Representatives and 33 Senate seats (one third of the total), as well as a dozen governors' races, and the total outlay for Election 2012 may reach an unprecedented $6bn, equivalent to roughly $50 for every likely voter.

Right now, despite economic indicators that in previous elections would have consigned him to defeat, Mr Obama remains the favourite. Recent history suggests that incumbents who seek a second term usually succeed, and at the time of writing Intrade, the usually reliable political prediction market, gives him a 75 per cent chance of victory.

Since Herbert Hoover lost to Franklin Roosevelt in 1932, only two incumbents have been defeated: Jimmy Carter in 1980, when the opponent was Ronald Reagan, who arguably caught America's Zeitgeist more perfectly than any candidate before or since; and George H.W. Bush, whose misfortune in 1992 was to find himself up against the most gifted politician of his age in Bill Clinton, at a moment when Republicans had already held the Presidency for 12 years. The same rule of thumb operates in the US as in most other genuine democracies. When one party has been in power for a decade the electoral mantra is: throw the bums out.

But even if Romney is manifestly neither a Reagan nor a Clinton, a second Obama term is far from set in stone. The mood of the country is sour; 35 per cent of Americans say the country is on the wrong track, a distinct improvement from a year ago to be sure, but hardly a resounding endorsement of the status quo. The worst of the Great Recession may be over, but the recovery struggles to gather steam. Since FDR, moreover, no president has been re-elected when the US unemployment rate was over 8 per cent. At the end of August it stood at 8.1 per cent.

Further complicating matters is the electoral system itself. Presidents are not elected by direct popular vote (if they were, the 43rd president would have been Al Gore, not George W. Bush) but by the sum of 51 separate elections in the constituent states and the District of Columbia. Each of these in turn sends voters to a 538-vote electoral college, all committed to the winner of the popular vote in their state – except in the cases of Maine and Nebraska, which allocate electoral college votes to the winner of each congressional district.

The number of electoral votes is in proportion to a state's population. Thus the most populous, California, has 55, while the least populous, Wyoming, has just three. To win the presidency a candidate needs to win a majority, ie 270, of these super-electors. It is thus possible, though unlikely, that either Obama or Romney will suffer Gore's fate.

At the very least, electoral college landsides, as defined by one candidate winning 400 or more of the 538 votes, are no more. Once they were common; of the 10 elections between 1952 and 1988, seven saw margins of that size or larger – the biggest in 1984, when Ronald Reagan trounced Walter Mondale by 525 to 13, with the latter winning only DC and his home state of Minnesota.

But in an ever more polarised America, those days are over. In both 1992 and 1996, Bill Clinton failed to crack the 400 mark, while George W. Bush's two subsequent victories were squeakers. The 365 electoral votes amasssed by Obama in 2008 – a year when everything, from financial crisis and a desperately unpopular outgoing Republican president to Obama's personal charisma, favoured Democrats – may be close to the realistic maximum for either party. Certainly, it would be astonishing if Obama matched that score, five weeks hence.

In practice, this election will be fought and won in a dozen or so battleground states, most notably Ohio, Florida, North Carolina, Virginia, Wisconsin and Colorado, where the result is not a foregone conclusion. Right now Romney is trailing in almost every one of them, which explains why he is a 3 to 1 outsider in the race. His own shortcomings as a candidate, and his failure to provide a vision of where he wants to take the country, are one reason. No less important, Americans seem to accept that Obama, though shorn of his aura of 2008, could not have been expected to correct in a mere four years the profound economic problems laid bare by the financial crisis. He has not succeeded – but nor has he yet conclusively failed. A majority of likely voters appears ready to give him a chance to finish the job.

Such calculations of course could be turned on their head, by events abroad (a European financial collapse and Wall Street meltdown, say, or an Israeli attack on Iran's nuclear installations) or at home, most obviously a really strong performance by Romney in the three presidential debates, the first and most important of which takes place tomorrow. In addition, some scandal or huge faux pas could undo Obama.

But such scenarios are increasingly hard to imagine. The President is a cautious, disciplined politician. And even a stellar debate performance is no guarantee of victory, as John Kerry found out in 2004. Separately, Obama is more trusted than Romney on national security, and a foreign crisis could actually help him. Time is also running out. Early voting in some states has already started, and in others will do so in a week or two. Polls suggest that at this late stage, in today's polarised political climate, fewer people than ever (maybe 5 to 7 per cent of voters) are genuinely undecided.

And while Romney may be a lacklustre candidate, his cause has not been helped by his party. Not only do Republicans sometimes seem to inhabit an alternative universe, on tax policy, abortion and other social issues. Collectively, they are growing steadily whiter, older, more male and more conservative, when the country at large is becoming younger, more diverse and socially more liberal. Especially telling is Obama's huge lead (almost 20 per cent in swing state Virginia) among women.

These trends could also determine the outcome of the Congressional elections. Until recently it had seemed that, even if re-elected, Obama would have to work with a Republican-controlled Congress, with his opponents retaining the House and making the net gain of four seats to secure a majority in the Senate. Again, however, this may no longer apply.

In several close-fought Senate races, the Democratic candidate is now ahead. And so unpopular is the Republican majority in the House, with its intransigent Tea Party bloc increasingly held responsible by voters for the gridlock in Washington, that there, too, Democrats conceivably could snatch back control.