Secondly even if 20% of Democrats jump ship in PA with Obama (which I don't think will) he makes that up by running better with Independents than Kerry did. To suggest at this point McCain has a better chance of taking PA than Obama does of taking Colorado is just hackish silliness.
But the independents that go for McCain will neutralize that.
Throw into that calculation conservative turnout for McCain against Obama will be nothing like it was for Bush against Kerry in '04 and that Obama is likley to take a much bigger chunck of the Independent vote than McCain overall and things get a little more complicated... a big reason PA was so close last time and Bush was pushed over the top in OH was the conservative and evangelical vote comming out in force and voting by a massive (and i mean massive!) margin for him, is McCain likley to repeat that?