Toward the end of a great column on Iran, Fareed Zakaria's references the convenient truth about Iranian senior policymakers -- they want improved relations with the United States, citing the story of James Dobbins, the only person who's ever been actually sent to try to cooperate with Iran: "Dobbins says the Iranians made overtures to have better relations with the United States through him and others in 2001 and later, but got no reply. Even after the Axis of Evil speech, he recalls, they offered to cooperate in Afghanistan. Dobbins took the proposal to a principals meeting in Washington only to have it met with dead silence." Gareth Porter has reported on Iranian overtures from as recently as 2003, and you can read Flynt Leverett on the whole history of this sort of thing.
Unfortunately, even the politicians who do favor more robust diplomacy are so concerned with making themselves sound tough that they wind up obscuring this point. The case for diplomacy, however, isn't that Hillary Clinton or Barack Obama could use the evil eye on Ayatollah Khameini and make him back down. The case for diplomacy is that US-Iranian conflict is a negative-sum enterprise, that US-Iranian cooperation would be a positive-sum enterprise, and that recent diplomatic history suggests that important elements in Teheran recognize this reality and would welcome a diplomatic opening. Can we be sure that verifiable nuclear disarmament is a price they'd be willing to pay for normalization of relations? We cannot, but it seems likely. And if the US and Iran were settling our differences over the nuclear and regime change issues, then suddenly we'd find that we both share an interest in stabilizing Iraq and Afghanistan and checking al-Qaeda. But as long as conflict over nukes and regime change continues, neither side can afford to let the other get the upper hand in either country, probably dooming both to chaos.