Here's the thing I keep coming back to about this election and what it will take to win it. It's a point that Hillary Clinton grasped, albeit too late to save her. And it's an open question about our own nominee and how he will approach the next 116 days.
Most people who would consider voting Republican in this (or any) election either like McCain, grudgingly respect him, or are hard-core Republicans/conservatives who ought to be persuadable for any Republican, even McCain. But none of those groups is going to be fired up with positive enthusiasm for the guy or his platform. On the conservative side, he's got folks who need regular reminding why they should vote for a guy who has butted heads with them so many times; on the moderate side, he's got people who are OK with him but are feeling like maybe the new guy from the other party deserves a shot. McCain has the experience and the biography, he is good on some issues (your mileage may vary as to which ones), and has some good ideas (ditto), but very few people are super-enthused about the things he is promising to bring to the Oval Office. Reassured, perhaps, but not enthused.
At the same time, McCain's opponent is not Generic D but rather a left-wing extremist with no experience, horrible, tried-and-proven-failure ideas and terrible judgment in friends, supporters and staff. That ought to frighten moderates and conservatives alike when they contemplate giving him the car keys. McCain's path to victory, then, is in collecting the people who like him, the people who respect him, and the people who can force themselves to tolerate him, and persuading them that an Obama presidency would be a disaster for the nation.
But McCain can only do that consistently and effectively if he, himself, believes that Obama would be a disaster for the nation.