Compared to Iraq, Iran is a far larger prize (it's literally larger, too, in size and population). You see, even before the invasion began, Iraq was already reduced to a shadow of its former self by genocidal UN sanctions, incapable of trying to become a contender for regional leadership (which was the specter posed by its invasion of Kuwait, prompting the Gulf War) again. Not so with Iran. Iran is modern, its populace well educated, its revolutionary philosophy -- the heritage of Ali Shariati now personified by Mahmoud Ahmadinejad -- appealing to not just Shiites but other Muslims elsewhere groaning under the oppression of pro-American regimes (note, in particular, that the oil-rich Gulf states have large Shiite populations, with Bahrain having a Shiite majority). You might say that the invasions of Afghanistan and Iraq were to surround Iran, the beginning of a siege for regime change, the regime change Washington wants even more now than before the election of Ahmadinejad.
Western leftists ought to do what we can to help Ahmadinejad survive this siege. That's not just our moral duty to the Iranian masses who voted to cast their lot with him. In the process of survival, internal and external circumstances may eventually compel Ahmadinejad to choose between his political, economic, and social agenda on one hand and the structure of the Islamic Republic on the other hand (which are sure to come into conflict) -- then, we'll see what stuff he is made of. Depending on the circumstances and choices he will make under them, what we will have is the beginning of an epoch-making change in the entire Middle East, comparable to the rise of the Bolivarian Revolution in Venezuela and gradual formation of a regional bloc in which the left (led by Hugo Chavez) and the right (led by Lula), leaders and masses, contend with each other over democratic control of natural resources in particular and production in general. Given where most of the world's oil lies, change in the Middle East is even more urgently needed than change in Latin America. Who better to initiate that change than a Bolivar in Tehran?
Iran is the key -- both for the ruling class and for us.