Iranians go to the polls tomorrow after one of the most lively and keenly fought campaigns since the Islamic revolution. The main choice, between the populist incumbent, Mahmoud Ahmadinejad, whose rabble-rousing rhetoric brought him victory last time around, and Mir-Hossein Mousavi, a former prime minister hailed by Western observers as a reformist, suggests that the revolution has come of age.

Two-thirds of Iran's population, and one third of the electorate, are under 30; the only world they have known is that of the Islamic Republic. But that does not mean that they are not interested in change or aware of other possibilities. The campaign has shown the extent of their engagement – on both sides of the argument. The campaign has brought hundreds of thousands out on to the streets in rival rallies.

Mr Ahmadinejad has on his side the young cohorts of the revolution, and the country's poor. Mr Mousavi has attracted a following among the middle class and women, thanks in part to his promise to reform laws that are, as he put it, unfair to women and the high profile taken by his wife. The position of women has become an issue in this campaign, but so has economic management. Mr Ahmadinejad has, not unreasonably, been challenged about why the economy is not in better shape, given the high oil prices of recent years. Mr Mousavi has traded on his reputation as a competent economic manager.

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The Presidential election in Iran matters. That Iran is a theocracy, with Ayatollah Khamenei as Supreme Leader, does not alter the fact that when Iranians go to vote, they will have a real choice, and one that will determine much about where Iran goes from here. The enthusiastic campaigning is a good sign.

A victory for Mr Mousavi would be welcomed abroad, especially in the United States, as easing the rapprochement President Obama has offered. But Mr Ahmadinejad showed, by publicly supporting a retrial for the US journalist (who was subsequently released), that he was tempted by Mr Obama's overtures. The election offers a chance for whoever wins to use his mandate to re-engage Iran in the wider world. It is a chance he should not pass up.