2008 elections? Easy- McCain in a landslide, hands down. Reason is if Iraq goes bad McCain runs on the "Get US out" platform, where he takes advantage of his "outsider" reputation in relation to the Republican party to come across as equal to whichever Dem they run in their "get US out" campaign.
And if Iraq goes well, and we actually establish something positive in the Middle East, then you'll see McCain and Bush grinning together on the podium, where Bush will endorse McCain's bid for the 2008 Presidency.
McCain is in a win-win, and it'll probably be until 2012 before the Dems have a real shot at winning the Presidency.