It's been widely observed that, lacking any weapons and on their own, the Mousavi protesters may not be able to bring down a deeply repressive regime committed to holding on to power. Some of the security forces would have to change sides for protests to become revolution. The IRGC has gained so much from the current regime -- indeed, some would say, it is the current regime -- that it seems an unlikely candidate. This is, in fact, why repressive states create parallel armies like the IRGC in the first place. Remember, the Revolutionary Guards are not the Iranian army, nor are they the Iranian police. Twitter-based rumors suggest that the conventional security forces may not be pleased with apparent theft of the election and subsequent crackdown, and presumably, they're less loyal to the supreme leader than the IRGC. Nazi Germany had the SS to keep the army in check. Iran has the IRGC. Khamenei and Ahmadinejad are hoping that if they wait long enough, the protests will dissipate on their own. The protesters, on the other hand, seem to have no clear road to victory. So long as the Guardian Council can't be relied on for a fair recount -- and presumably, it can't -- then there is no apparent legal, procedural avenue for Mousavi to become president. It's now pretty hard to imagine a scenario in which Khamenei backs down, inaugurates Mousavi as president, and everything goes back to normal. This means that either Ahmadinejad will probably remain in power, or he'll probably fall from power in an extralegal, nonprocedural fashion. That is, the people with the guns stick with Ahmadinejad, or enough of them desert him to give Mousavi a literal fighting chance. And if Iran is looking at a fight between a loyalist IRGC and a dissident army, that sounds like civil war, and the furthest thing from normal.
link: Who hates who in Iran | Salon News