Yes,
there is a compounding effect now, and frankly McCain is his worst enemy - namely his antics, the personality of McCain - that is some of thing that drive people away from him. McCain is simply not a likeable person. He's not as awful as Giuliani, though.
George W. Bush had one REALLY important thing going for him - he is an always jovial person, although being an idiot.
GWB readily admits that he is not the smartest cookie, and he does all kind of stupid, goofy things - but he still has some charm.
McCain is like a geriatric nasty neighbour with a pee-stain on his trousers and smelly... kid-hater and nasty.
That is psychologically a big difference as well, and as more debates go by, people perceive and compound these impressions. The ugly grimaces and bad tone from McCain is not appealing to anyone - no matter how hot Palin is and attention is successfully drawn to her.