We can only imagine the burden of high expectations borne by Barack Obama.

One term as a junior Senator — most of it spent seeking the presidency — and an eight-year stint in the Illinois Senate is the sum total of his political record. State senator is a bit more than the equivalent of a local councillor in this country; a bit less than a Member of the Scottish Parliament. He was a clever enough academic to be president of the Harvard Law Review and wrote a beguiling memoir at the age of 33.

On that slight prospectus, he secured the support of the voters of the United States and, by proxy, the non-voters of the rest of the world, to assume leadership not just of the most powerful nation but of the entire planet. If the hopes that Americans have of him are high, those of the rest of the world, from Azerbaijan to Zimbabwe, are higher still.

In Britain, as our poll today suggests, fully three-quarters of the population say they are “confident that Barack Obama will live up to the expectations people have that he will be a force for good in the world”.

Talk about the audacity of hope. And this in a country whose defining political experience in recent years has been that of high hope disappointed.

No one wants to push the parallels with Tony Blair too far — not this week of all weeks.

But some of them are inescapable. Blair, too, was “a party of one”, as John Heilemann described Obama in New York magazine last week.

There are other parallels. When Parade magazine published Obama's sick-bucket letter to his daughters last week, it sounded a faint British echo.

Believe it or not, it was John Major that started it. As a New Year stunt the year before the 1997 election, Major wrote a letter to his children, aged 20 and 24, that was published in the Daily Express. “We must encourage saving,” it said, among other things. Such was the state of New Labour's rapid response, no matter how crass the provocation, that the next day's Daily Express carried a letter from Blair to his seven-year-old daughter, Kathryn, in which he told her: “Never let anyone say that there is a better place to live than Britain. But never let others tell you that Britain cannot be better.”

More obviously, Obama is a lawyer with an unusual gift for bringing people together. Like Blair, he is good at appearing to take head-on the case against him and to turn it round.

Not that Blair ever tried anything quite as showy as Obama's speech about religion and politics in which he took as his text the statement made by Alan Keyes, his Republican opponent in the election to the Senate in 2004: “Jesus Christ would not vote for Barack Obama.”

At the time, Obama ignored it and he went on to defeat Keyes by 70 per cent to 27 per cent. But he could not let it go, Obama told a church audience two years later: “I had to take Mr Keyes seriously.”

Despite trouncing his fundamentalist opponent, Obama felt impelled to go back and try to win over the few voters that had resisted his charm. “If we truly hope to speak to people where they're at,” he said, “then as progressives we cannot abandon the field of religious discourse.” He went on to argue that both fundamentalists and secularists should use language that united rather than divided.

That was the skill of which the British electorate tired after six years. In 2003 they gave Blair one last chance — with the case for the invasion of Iraq — that turned out to be a double-or-quits gamble that Blair lost.

When no weapons of mass destruction were found, much of the British public agreed with Gordon Brown when he told Blair in 2004: “There is nothing that you could ever say to me now that I could ever believe.”

It is not inevitable that Obama will follow the same trajectory. There are great differences: the checks of America's size, the balances of its constitution; above all, the state of the economy, which provides Obama with an external threat against which to rally.

And Obama marks a break not just with the anti-intellectualism of George Bush but with the very assumption, burned deep into New Labour, that voters are suspicious of intellectuals.

True, his big set-piece speeches are assemblies of soundbites, brilliantly delivered, but his best speeches contain real persuasive argument that make no patronising assumptions about either his audience's vocabulary or its attention span.

There is something intrinsic about the cycle of hope and disillusion in democratic politics. What politician can be expected to say: “It won't be that good, you know”? But voters have a responsibility too — as do we non-voters. Americans would be entitled to say to us: “Ask not what Barack Obama can do for you.” European countries should, for example, stump up troops and cash for Afghanistan — Obama taking the European consensus view that, unlike Iraq, this is a war worth fighting.

However glad we may be to see the back of George Bush and Dick Cheney, we all know in our heads that Obama cannot meet all the sky-high expectations read into him.

But, even when he does make mistakes, as he will, the world will be a better place. So who is being unreasonable: us, or him?