For a start, being British, we are cynical about anyone so persistently cheerful. And, after the Blair premiership, it is not surprising that most of us are deeply suspicious of clean-cut young men, with toothy grins, bearing gifts.

Millions of people around the world will be pondering whether the Democrats' candidate is too good to be true when tonight, barring disasters, he accepts the nomination of his party to contest November's US Presidential election.

Much about him is reminiscent of a young Tony Blair. He is a smart lawyer, with a smart lawyer-turned hospital boss wife (better paid than him, too) who isn't afraid to capitalise on her own working-class background to create political capital. He is undeniably vital, exciting and a natural reformer, promising a bright, shiny new future.

Americans are far more likely to squeal with delight than Brits when the Obamas snatch a quick kiss. We still haven't quite got over Cherie Blair's appearance in her nightdress after the 1997 election, and David and Samantha Cameron's whispered "I love you" on the conference podium last autumn. It makes us squeamish to think of politicians being intimate.

With the Obamas, the evidence is right there in front of us, presented at every opportunity. Their two daughters, Sasha, seven, and Malia, 10, appear every bit as self-possessed as their parents; far more so than the Blairs' children when their dad entered Number 10 for the first time.

There wasn't a mother whose heart didn't go out to those awkward boys and shy, chubby Kathryn. Compare – we can't help it – Kathryn to Malia Obama, with her dreadlocks, fist bump and the family brain whirring beneath the streetwise insouciance. If you think Chelsea Clinton is scary, you ain't seen nothing yet.

As well as wondering what we would make of a politician like Senator Obama, I seem to be spending a lot of time wondering what Gordon Brown must be thinking of him. At 57, he is only a decade older than Obama's youthful 47, but he seems to have come from a different generation.

It might have been because it was the Bank Holiday, but I noticed Gordon going for the open-necked shirt look over the weekend. It is a style perfected by his potential American counterpart. I wonder if the younger man's laid-back sartorial approach is rubbing off just a teensy bit on our own PM? I know it is superficial, but modern politics owes a lot to image.

There is a lot more to it than leaving the tie in the wardrobe though. While many British voters find Brown's upbringing as "a son of the manse" distinctly un-engaging, Obama's amazing journey from growing up on food stamps as the son of a single mother, to the gates of the White House, can't fail to capture the imagination, whatever your political affiliations.

His wife made much of his "American story" in her convention speech earlier this week. His background has propelled him this far, but will it help or hinder him on the next step, through those gates and into the Oval Office?

The irony is that the things which have made him into such a hero could be his very undoing. For youth, read inexperience. For fighting on behalf of under-represented minorities, read Michelle Obama's bitter attitude to America. For being a modern professional aware of work/life balance, read inattention to duty while on holiday in Hawaii.

Barack Obama isn't the Leader of the Free World yet, and nobody could argue that the guy didn't need a break, but his response to the Russia/Georgia conflict came too late. And when it did come, it wasn't robust enough. If ever a situation was guaranteed to play right into
his Republican rival's hands, this was it.

Nobody has ever accused John McCain of being too good to be true, but he represents a safe pair of hands for so many Americans, a battle-hardened finger on the nuclear trigger and a natural successor to George W Bush.

Obama's struggle is fuelled by the controversy between himself and his former running mate, Hillary Clinton. Although she has spoken in favour of him this week, many voters remain disgusted that she was sidelined in the vice-presidential nominations.

A recent CNN poll indicated that 27 per cent of Mrs Clinton'ssupporters will not vote for Obama in November, and are planning to switch their allegiance to McCain.

These are votes which Obama simply cannot afford to shed. For all the inspirational back-story and the slick presentation, there is now a very real possibility that he could lose because of political misjudgment. Too good to be true, indeed.

From this side of the Atlantic, the McCain camp's anti-Obama slogan, "A mile high and an inch deep", has a strangely prophetic ring about it. It has been a long time since the Brits could tell the Americans anything, but right now, it might just pay to take note of a bit of history from across the Atlantic.