As America's government prepares to take a sudden and historic leftward turn, this might seem an odd moment to ponder what a conservative country it is.

On Wednesday morning, unless the political equivalent of a giant meteorite hits Earth before then, Democratic supporters in America, in happy union with almost the whole of the civilised world, will be singing hosannas to the new President-elect. They will expect the Obama proto-administration and the expanded Democratic caucus in Congress to press hard to implement quickly their agenda of wealth redistribution; a tougher and broader scope of government regulation; and an enthusiastic embrace of foreign policy multilateralism.

But the new rulers and their allies overseas would be well advised to tone down the rhetoric, play down expectations and rein in their wilder tendencies. The easiest mistake for the world to make would be to start believing the Left's own propaganda: that a vote for Barack Obama and for a Democrat in Congress on Tuesday is a vote to transform the country into a kind of social democratic paradise.

Perhaps the best supporting evidence for this claim is the likely outcome on Tuesday. Though there's no real doubt about who will win, the presidential election still looks like being a closer contest than it has any right to be.

Consider the objective facts of political life in late 2008.

You have the end of a two-term presidency, when the country is always hungry for change. You have an economy that has slipped, with almost exquisite political timing for Democrats, into what looks likely to be a deep recession. You have a global financial crisis the like of which has not been seen in 75 years.

You have deep dissatisfaction with America's standing in the world, widespread (albeit post-hoc) disapproval of the biggest Republican-driven foreign policy event of the past ten years, the decision to go to war in Iraq and something bordering on alarm about the prospects for success in the other continuing war in Afghanistan.

You have a Republican Party that has displayed an unpardonable level of incompetence, inadequacy and venality in the past four years. From the handling of Hurricane Katrina to a steady procession of members of Congress in legal trouble: a rogues' gallery with a record of crimes and misdemeanours ranging from taking bribes to soliciting sex in public toilets. Just this week, in a timely reminder of the toxicity of the Republican brand, Ted Stevens, of Alaska, became the first sitting senator to be convicted on felony charges since 1981.

In the campaign, you have a Democratic candidate who has raised twice as much money as the Republican, a jaw-dropping $650million, enabling him to blanket the country with advertisements and, culminating this week in an extraordinary 30 minutes of nationwide TV on the networks.

You have a fractious and visibly enfeebled Republican campaign that seems to be unable to suppress a disturbing psychological tendency towards self-harm. And, of course, you have media that have managed to exceed themselves in their obeisance towards the left-of-centre candidate, raising to new levels of absurdity their claims of objectivity.

What, in these circumstances, would a scientific model predict as the winning margin for the Democratic presidential candidate: 10, 15, 20 percentage points? In fact, as of yesterday, Mr Obama seemed to have a solid but by no means overwhelming advantage of between 5 and 6 percentage points.

If this were a football game, it would be one played on a field tilted at an angle of about 20 degrees, in which the teams did not change sides at half-time, and in which the one playing downhill had twice as many players on the field as its opponents, who, to make things a little bit more interesting, have bound their goalkeeper hand and foot to one of the goalposts. The final score? 3-2, after extra time.

Or put it another way: it has taken a mismanaged foreign policy that almost lost a war, a botched emergency response that almost lost a city, a Republican Party that almost lost its soul and an economic crisis that almost lost the country's financial system to break the Republican stranglehold on the White House.

The usual objection here is that Mr Obama would be doing much better if there weren't so many racists in America. That, in his own words, too many Americans have been prodded into worrying that he “doesn't look like all those other presidents on the dollar bills”.

I've been through this argument before. The main problem with it is that it has the question about Mr Obama's race almost precisely the wrong way round. In fact his skin colour has assisted, not hindered, Mr Obama in making the case that he represents change.

This is not to denigrate his remarkable talents; a great speaker, a lively mind, a remarkable self-discipline and organisation. It is to say that, looking at his actual record - a solid and wholly predictable left-of-centre stance on all the big issues - it is precisely that he doesn't look like any of America's other presidents that makes his claim to be something new most plausible. In every other political respect he is not much different from failed Democratic presidential candiates of the past.

Of course none of this is to deny that Americans will be voting for change on Tuesday. After the past eight years who could possibly blame them? But I sense that the change they want is for a welcome period of competence and decency. They would rather have a government that works than a new paradigm for the organisation of American society, a radical redistribution of income and wealth or a dramatically expanded public sector. And they certainly won't be voting for a “can't we all just get along?” foreign policy that subjugates US national security to the negotiated outcomes of debates in the UN Security Council.

What's more, I suspect Mr Obama is easily clever enough to know that this will not be his mandate either. The real test of a President Obama's political skills will be his ability to recognise the limitations placed on him by what is still a deeply conservative country, and his willingness to stand up to those in his party who will be impatiently pressing him to go much farther.

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