That was the greatest mid-term defection in history ~ most analysts have used that as an upper limit to the Conservative vote assuming all Conservatives chose that as an opportunity to show displeasure. Actually, that was just the hard-core Christian vote who concern themselves with such matters in politics ~ there are about 17 million of thems. The other 9 million were simply voters who prefer to not vote in anything but the presidential race.
The fall off from W to McCain is no where near the 17 million hard core serious minded Christian numbers ~ it's just 4 million. That's the Republican left.
I am not so sanguine. Just because fewer than all the conservatives left, does not mean that some of the conservatives did not leave.
Why would a liberal vote for Bush yet not vote for McCain? That to me makes little sense. What makes to me more sense, is that some of the conservatives chose not to stick with McCain.