All sides are behaving as if Obama has it in the bag. And yet, and yet ...

A little shudder went through me every time they said it. "President Obama's first priority will be this," they would begin. Or, "The Obama administration will find that ... " Travelling in the US last week, I lost count of the pundits, experts and Democratic insiders I heard speak with such confidence. And each time they did, I felt the urge to mutter: "Presuming he wins."

Call it superstition, but it's one rooted in years of unhappy experience. After the British general election of 1992, or the Al Gore and John Kerry defeats of 2000 and 2004, the centre-left has surely learned that you don't count your chickens till they're hatched, squawking and deep into middle age. Which is why I won't believe an Obama presidency is possible until the morning of November 5 - and even then I'll wait till inauguration day on January 20, just to be sure.

This is not fully rational. But who said elections are rational? They call it political science, but there's all too little that's scientific about it. Otherwise you could weigh the data and draw definite conclusions about what is about to happen. But politics doesn't work like that.

Rationally, I know, every indicator points to a clear victory, if not a landslide, for Barack Obama. He is ahead in the polls, including in most of the key battleground states that will determine the winner. John McCain has now given up on Michigan, Iowa, Colorado and New Mexico - and is betting everything on taking Pennsylvania, a state even Kerry managed to hold four years ago. McCain's path through the electoral college is now narrow and perilous.

What's more, he is low on ammunition. He has just $47m to spend, while Obama raised more than three times that amount, breaking every record, last month alone. In the air war of TV advertising, McCain is coming under saturation bombardment: for every ad he runs, Obama has three, four, even eight.

Just as important, Obama is outgunning McCain on the ground. He has field offices, staffed up and teeming with volunteers, in the unlikeliest of places - small towns that cannot remember the last time anyone remembered them. Fivethirtyeight.com's Sean Quinn has been dropping in on those and on the McCain operation and describes a stunning contrast. Not only do Obama's offices outnumber McCain's by four to one, but the latter are often empty or near-empty, filled with lines of unused phones, just one or two people making calls. Quinn added up the number of McCain volunteers he had seen in six states - and it equalled the number working for Obama in the single town of Durango, Colorado.

The intangible signs are just as positive for Obama. The endorsement by Colin Powell got all the attention, but the Democrat has also won the backing of conservative newspapers, from the Houston Chronicle to the Idaho Statesman. No one pretends such endorsements swing elections, but they tell you something about the prevailing wind.

And yet I still won't say this election is over. For one thing, there is no precedent for this contest: an African-American nominee for a major party is a first. We simply don't know if the notorious Bradley effect - whereby more voters tell pollsters they plan to back black candidates than actually vote for them - will bite on November 4. Last week senior Democrats were telling me to look at the Tennessee senate race of 2006. Pre-election polls showed the African-American Democrat, Harold Ford, behind by three points - and he lost by three points. The polls got it right; ergo, the Bradley effect is dead.

But this is a presidential race; it's different. A pessimistic gut instinct tells me that older, white voters across Appalachia could break for McCain, putting Ohio and West Virginia in his column. Perhaps McCain thinks the same and that's what's prompted him to bet everything on Pennsylvania. With a relentlessly negative message in this last stretch, powered by robocalls suggesting Obama pals around with "domestic terrorists" and by fliers photomontaging Obama's face with that of Osama bin Laden or subtly linking him to 9/11, Republicans believe they can plant just enough fear that Obama is an alien, un-American stranger - "Not who you think he is," says one leaflet - to make November 4 a very long night. Meanwhile, if Democrats grow complacent, and young voters don't turn out, believing victory is already in the bag, those Republicans could be proved right.

But let's say I'm too much of a worrier and that we should trust the objective data - let's dare ask the question now exercising the chatterers in Washington and beyond. What would an Obama presidency be like?

The first, depressing thought is that, if the result is close 13 days from now, Republicans will seek to challenge Obama's very legitimacy. That sounds far-fetched: after all, George W Bush only won in 2000 thanks to a single vote on the supreme court and the Democrats did not hesitate to salute him as the commander in chief. But Republicans play by different rules. Recall the treatment meted out to Bill Clinton. He won fair and square in 1992 and again in 1996, but that did not stop Republicans using every means to cast him as essentially unfit for high office - culminating in the impeachment effort of 1998. They will surely follow the same approach towards a President Obama. Indeed, they're doing it already: last week McCain campaign manager Rick Davis claimed a "cloud of suspicion" hangs over the election, because of Obama's links to Acorn, a grassroots group accused of trying to register bogus voters. That sounds a lot like a party preparing either to make a legal challenge to a close result - or to brand the eventual winner as illegitimate.

If Obama wins big, that will be a harder case to make. Then what? Some Democrats are growing excited at the prospect of what could be the strongest, most progressive administration since the Lyndon Johnson landslide of 1964. That may not be wholly delusional. On current projections, the Democrats are on course to win both the House and perhaps a filibuster-proof 60 seats in the Senate. Obama would have the muscle to drive through a truly radical programme.

Still, the voice of caution nags. First, the precedents are not encouraging. Both Clinton and Jimmy Carter were greeted by a fully Democratic Congress - and both got snarled up early. If the smoothness of his campaign is any guide, Obama will be a savvier operator than either of those two predecessors. But the danger is real.

Second, Obama could be constrained by a simple fact: the cupboard is bare. After years of profligacy and Bush's ballooning deficits, there is too little money around for a raft of government projects that would cost dear.

And yet, Obama has made clear that he holds the Keynesian view that a recession is precisely the time to start spending money. What's more, the banking crisis has so thoroughly discredited the laissez-faire approach to free markets, that the public mood is conducive to a shift leftward. One US historian suggests Americans only allow a dramatic expansion in government after a great rupture: FDR was preceded by the crash of 1929, LBJ by the Kennedy assassination of 1963. If that's true, then Obama might indeed prove to be, as Powell predicted, a "transformational president".

See, even I'm at it now - getting way ahead of myself. Of course Obama has the potential to do all kinds of great things, but we're not there yet. First, he has to win. [e-mail]