Would President John Kerry have pursued a different course with Iran?  Dan Drezner says 'no':

The approach the Bush administration has pursued towards Iran -- multilateralism, private and public diplomacy, occasionally deferring to allies -- is besotted with the very tropes that liberals like to see in their American foreign policy. I'm still not sure what the end game will be with regard to Iran, but to date I can't see how a Kerry administration would have played its cards any differently than the Bush team.

I have to disagree slightly with Dan here, only because both John Kerry and John Edwards told us repeatedly in 2004 what their policy toward Iran would be: direct negotiation and the offer of a "grand bargain." It's highly likely we'd be in the same place with Iran today under a Kerry administration, of course, except we would have travelled a slightly different policy road - one a bit more besotted with liberal tropes.