Comes from Saturday's NYT:
McCain's advisers were divided, for example, over a speech he gave on nuclear security policy in Denver in May. Two Republican pragmatists who advise McCain, former Secretaries of State Henry Kissinger and George Shultz, supported a call in the speech for a nuclear-free world, an idea they endorse as part of a "Gang of Four" of national security statesmen. But other advisers to McCain were opposed to the idea because, in their view, nuclear weapons act as a deterrent against an attack on the United States and its allies. In the end, Lehman said, McCain made the call in favor of a nuclear-free world.
"He wanted to do it," Lehman said. "That position is McCain's position. It's not a cabal of Kissingerites or a cabal of neo-cons."
My uneducated guess is that McCain sided with the prags because he liked the idea that calling for a world free of nuclear weapons is the right thing to do. (Though he couldn't have been averse to the realpolitik argument--that nuclear weapons deter the United States and our allies more than they protect us.)
The real question is if McCain cares about the issue enough to force reform past conservatives, who will try to hamstring the process. Get someone like Bolton back at the State Department and we'll have non-negotiable proposals, legal smokescreens, devastating leaks, and policy gridlock until 2012.
Update: Some commenters took issue with Elisabeth Bumiller's term "Gang of Four." Gauche comparisons between Henry Kissinger and Mao's wife aside, the term--and comparable ones like "The Four Horsemen"--refers to the authors of a 2007 op-ed that shifted the terms of foreign policy debate. Check out this piece for the backstory.