Technically, there are four candidates running for President, but CBS News correspondent Elizabeth Palmer reports, it's really a two-horse race.
With the main candidates running neck-and-neck on election eve, the excitement is just electric. It has spilled onto the airwaves, into political rallies and certainly onto the street.
Wednesday night - like every previous night this week - the streets of the capital were overflowing and traffic was gridlocked for miles amid a sea of green.
Green is the color of opposition candidate
On the other side: President Mahmoud Ahmadinejad's supporters just use Iran's flag as their emblem.
To them, the President is a working-class hero devoted to helping the poor.
"Why do you support Mr. Ahmadinejad," we asked one supporter at a rally. The answer was direct: "Because he's done everything the people want."
The candidates faced off in televised debates - a first in Iran - and things soon got nasty.
Ahmadinejad was called a liar and a would-be dictator. Mousavi was accused of links to corruption.
The hostility hasn't seeped into the nightly street rallies, but the sheer number of opposition supporters has got the authorities rattled ahead of an election that is simply too close to call.
A warning has been posted on the Web site of the country's powerful Revolutionary Guard - that any attempt at a popular revolution will be crushed.
There won't likely be any large street rallies Thursday, or campaign events. Iran's election law prohibits campaigning on the day before an election - the calm before the storm.
The election results could come in as soon as Saturday, and the real question is what happens next. Which ever party loses, its supporters will likely be embittered and could potentially cause disruption in the days and weeks ahead.
As for the potential for real change in Iran's public and foreign policy, even if Mousavi topples the hard-liner from power, it's far from certain.
In Iran, the President is the highest elected official, but the... and whoever wins Friday's vote, essentially works under him, and his subordinate clerics.