Personally, I believe that America has turned against the Iraq war for the same reason they turned against the soldiers returning from Vietnam: We're losing.
I believe that principles have little or nothing to do with why a significant portion of people who were originally for the war in Iraq have turned against it. A quarter million or a full million civilian deaths, half of which are children, don't matter--not even enough to try and quantify. The fact that we've found out, and the world has found out, that we U.S. Citizens water-board people, torture people, doesn't matter. What matters to some of the more shallow among us, is that we are losing.
If McCain can show us winning in Iraq--that is, killing more enemy than enemy kills us-- even if we won't actually win anything we point to, public sentiment will move the other way. The majority of the American public will support the war again. Iraq's democracy might look like Cuba's in that there is only one candidate to vote for (no democracy at all) but eventually that'll be enough of a win for American historians to say the Iraq War was successful-- and have George H.W. Bush go from presidential disaster to presidential hero.
