Using the CNN Electoral Map Calculator, you can see for yourself that it is hard to imagine a scenario where McCain can win without Pennsylvania and Ohio. Based on the map below -- which is still an uphill fight -- McCain would just edge-out Obama with 270 electoral votes to Obama's 268.
I've tried to come up with a somewhat realistic scenario here for how McCain could plausibly win. As such, I am giving Obama states such as Colorado, Nevada, and Missouri. I also think that McCain will win Florida, North Carolina, and Virginia -- though he has some ground to make-up. (You can play with the calculator and invent your own scenarios. For example, it is possible that McCain could lose Virginia, but still win by delivering Nevada and Missouri.)
... Regardless, in my estimation, the election will be determined by the 41 electoral votes in Pennsylvania and Ohio. Most experts agree; it is hard to come up with a scenario that has McCain winning without these two tough battleground states.
Note: Based on the above map, losing only North Dakota's 3 electoral votes would flip the election to Obama...