What a spectacle this election is. It has galvanized young people and polarized the country.
Two main candidates – President Ahmadinejad (left), a hard-line conservative, against Mir Hossein Mousavi (right), a mild reformer. In admittedly unreliable polls, they are neck and neck.
President Ahmadinejad's well-oiled political machine will make sure his mostly poor and working class supporters turn out in droves.
The opposition candidate can rely on a huge, largely middle class base.
Mousavi's organizers hope he'll also benefit from a big protest vote — politically-cynical Iranians who wouldn't normally bother to turn out, but will make the effort this time to cast a ballot against the President, whom they loathe as an evangelizing rube.
The wild enthusiasm of millions of 20-somethings for Mousavi has been moving to watch, but they are probably in for a big disappointment, even if he wins. He bills himself as a "reformer". But what he means by "reform" and what many Iranian young people mean by "reform" are worlds apart.
As President, his policies and power would be limited by Iran's powerful Islamic clergy. Realistically, he would probably allow some mildly dissenting newspapers to re-open. He may reign in the so-called Morality Police who can arrest men and women for un-Islamic behavior in public (the sin can simply be a young woman allowing too much hair to peep out from under her headscarf.)
But the list of things he won't be allowed to allow is much longer: real freedom of speech; letting women dress as they please, making Iran's shadowy judiciary transparent and publicly accountable.
On foreign policy, he would almost certainly take a more conciliatory tone than the confrontational President Ahmadinejad, and probably offers the best hope for improved relations with the United States.
Already — only half-way through voting day — it's clear that the turnout is massive. Officials have announced the polls will stay open for a couple of extra hours.
What's not so clear is what happens when the result is announced. Alongside the excitement in Iran today there's a current of tension.
No one knows how millions of frustrated, passionate voters will react when they hear their man has lost.