A big chunk of The American Foreign Policy Establishment came out in support of engagement with Iran:
Five former U.S. secretaries of state said on Monday the next American administration should talk to Iran, a foe President George W. Bush has generally shunned as part of an "axis of evil." [...]
The five — Colin Powell, Madeleine Albright, Warren Christopher, James Baker and Henry Kissinger — all said they favored talking to Iran as part of a strategy to stop Tehran's development of a nuclear weapons program.
Kissinger specifically came out in favor of negotiating with Iran "without conditions."
The fact that McCain occasionally talks to Henry Kissinger is occasionally offered as evidence that McCain is hearing from a diverse group of foreign policy advisers. The fact that Kissinger advocates a course that McCain has derided as naive and irresponsible should be a huge clue as to whom McCain actually listens.