Now what? Mahmoud Ahmadinejad, will no doubt want the four years of his new term as president to be as firmly under his control as the past four. That will be harder than winning an election which his opponents are convinced he rigged.

It is not a surprise that the stunning "green wave" which poured onto Tehran's streets last week, a circus of dyed and gelled green hair, waving banners and blaring horns, failed to dislodge a candidate who had the Supreme Leader's blessing. Iran, tantalisingly to its young, sophisticated population, offers enough democracy to give them real hope of change if they turn out to vote. Many democracies, which lament low voter turnout, would love to know what to do to get an 85 per cent turnout, but the answer is hardly elusive, even if it is uniquely Iranian. You give people cause to be furious with the incumbents, and persuade them, at least for a week or two, that their vote will make a difference.

Supporters of Mir Hossein Mousavi now feel bitterly that they have been cheated. They may have been. The storm of YouTube videos and blogs pouring out from Iran's cities make one telling point, about the implausiblity of the figures ever since the first results came in: Ahmadinejad was always in front by precisely the same margin, two to one, right across the country, even though his support was known to be weaker in the cities.

The fury won't make much difference. We should assume the election result stands. But it will be a surprise if Ahmadinejad can ignore the profound shift which the past few weeks have made in Iran. The clever move for President Obama is now to make him an offer which, if refused, serves to ram home Ahmadinejad's responsibility for Iran's isolation. That will give the "green revolution" the best hope of survival.

The passion for change displayed by Iran's urban youth, particularly its women, gives Ahmadinejad new problems, on top of his old ones, which were already serious. To keep the bedrock of his support, he has to keep his bargain with the poor - to pay them the benefits of Iran's vast oil wealth. But the slump in the oil price made that hard, and even though the price is now rising, food and fuel subsidies are a huge strain on Iran's budget. If the poll was indeed rigged and his support among the poor is much lower than reported, he will have a problem controlling that resentment.

Now, he must also decide whether to suppress the uprising of the articulate youth, who have used Facebook, Twitter and YouTube to such effect in this campaign. The whole world has seen them protesting in the streets of Tehran, and read their accounts of boiling frustration, of being "half alive" for the past four years, and in those giddy nights in Tehran last week, finally feeling completely alive. Many of them - including Mousavi's wife, a renowned academic - have written of how Ahmadinejad's repression seems to them a betrayal of the Islamic Revolution. Ahmadinejad may not mind if many of them carry out the threat to leave Iran, although that will leave the country poorer, narrower and more isolated. But he will not be able to crack down on Internet and mobile phone use without risking sustained uproar and a generational clash with the ayatollahs.

What should Obama do? It is prudent to assume that Ahmadinejad, who has given not an inch over the country's nuclear programme in the past four years, will press on with it. It is also prudent to assume that a nuclear weapon, should he choose to go that far, is within his reach within his next term. But even more than in the past, with Iranians hyper-alert to their isolation from the world, it is worth the US and others putting together a package of offers, and threats, to Ahmadinejad. It may well not be possible to get him to change by much. But it should be possible to put him in a position where, if he rejects the offers of the West, he has to turn around and face the real threat of vivid green banners in the streets of Tehran.

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