In the run-up to the first debate Republicans allowed their expectations to get unrealistically high. Then many of them freaked when Bush didn't measure up. I'm convinced much of the erosion in the polls since then is despondent people on the Right moaning, "Oh no, here we go again" and diving into the nearest fox hole!
I think part of the problem is how, with an event like this, we're helpless captives. We can't give Bush the right words, we can't mute an obvious political hack "undecided", we can't enforce the rules, we can't control the post-debate "Kerry won" propaganda from the "analysts" and so forth. It's kind of like watching the airplanes fly into the WTC towers. You can't stop them mid-air.