Iranian politics in the Islamic Republic aren't supposed to be interesting. Presidential candidates are vetted by the clerical Guardians Council, a rather hard-line body (some call it a "Clerical Senate"). All candidates are supposed to support the Islamic state, and the ultimate authority of the Supreme Leader. Theo-democracies are not terribly dramatic.
Except when they are. At worst, we're seeing a rather nasty internal debate in the regime, between Rafsanjani, now with reformist alliances, and Khamenei. Fortunately, for the rest of us, those spats have become very public, leading to massive rallies and a surprising, if temporary openness. The youth have rallied to Mousavi, whose wife has become a surprising asset, and the energy is in his camp.
Which might not matter, if hard-liners rig the vote. The Revolutionary Guards are warning against any "Velvet Revolutions," a clear shot at Mousavi, Karroubi, and their supporters. This strikes me as a most unwise path for the mullahs.
Fellas, let's play this out. You've seen the same rallies we have and have no doubt watched in horror the public debates. Now, you want to put it to rest and quash. Problem is y'all let it happen. Putting that genie back in the bottle now? Yeah, good luck with that.
After all, guys, revolutions can start over much less. And unlike Chinese authorities in 1989, I'm wondering if you have the ability to quash a movement, should it start, divided as you are. Think about it fellas. Sure, the nature of your theocratic regime may need to be tweaked slightly in response to a Mousavi win, but from your perspective, it's better than blowing the whole thing up, right?
Oh yeah, the world would appreciate it too.