"The United States is now the thread that binds Iraq, and it is clear that a serious unraveling of the situation would occur were this thread suddenly to be pulled away."
If this be the case, we are in a situation from which there is no exit. Does this make McCain right for saying he'd stay there for another hundred years? Does it make Obama wrong for saying we need to remove ourselves ASAP (and who knows what ASAP means)? Could a Democrat lose the 2008 election because the surge failed? What a huge irony that would be.
Two things seem obvious to me. The first might seem provocative, so I'll preface it by emphasizing that I'm not drawing a moral or methodological equivalency here, but simply a structural and functional one. Namely, as the glue that holds Iraq's disparate parts together, the U.S. is now playing the role that Saddam Hussein formerly played in Iraq, and we're playing it for the same reason that we were willing to tolerate Hussein for as long as we did: to contain Iran's regional influence. What's more, it's a role that has once again led us to ally ourselves with some unsavory and unpredictable characters, all of whom have their own agendas that don't always correspond to ours. And short of the improbable appearance of an Iraqi strongman in the the structural, functional (and moral) image of Saddam Hussein, it's a role that only we can play.
The second observation follows from the first and responds to GS's question. At some point, the decision of whether or not to withdraw from Iraq will be made not on the basis of a military or strategic calculation, but of a political one. It will boil down to whether or not the American people are willing to bear the continued burden of lives lost and resources depleted (as well as the opportunity costs associated with both) in order to hold Iraq together indefinitely.
The risk is that a political decision to withdraw will come with a strategic cost, ie. further unleashing Iran's influence, although I think that argument underestimates the kind of challenge an unraveling Iraq poses to Iran. But inasmuch as the key, I think, is to find a new strategic calculus for dealing with Iran that removes us from the zero-sum game that we are currently engaged in with Tehran, Obama might be the best-positioned candidate to respond to McCain's position, since he's provided the most clear-sighted alternative to current Iran policy. That is what the choice boils down to, though, and I don't think either candidate is shying away from it.