You’re over analyzing the situation.
If McCain wins PA, he’s going to win VA, NC, IN, MO, etc.
If McCain loses IN or NC, it will be a nationwide route. There is talk of 50 separate contests, but the states generally move in tandem with the national popular vote. There are exceptions of course, but I don’t think you’ll see that this year with PA.
For the most part, that tends to be how it works. But there's the separate question of resource allocation. If McCain is spending money to chase Pennsylvania, he's not spending that money in North Carolina, Virginia or Ohio. National trends are most important, but localized trends happen as well or else you wouldn't have local campaigns.