You bring up a critically important observation. Obama hasn't extended himself into Republican dominated territory. He's fallen back to defending Democrat dominated territory because Mitt has advanced his offensive and has him on the run.
In military terms, Obama is in a mad retreat. You can see it in everything his camp is doing and saying. They're going down, and they know it.
Let's be realistic. Obama has 358 electoral votes if he holds on to what won in 2008. The only state he had any chance of picking up was Missouri which McCain won by .13%. Why would Obama do anything but go to Missouri and the states he won by the lowest margin. Ditto Romney, why would he go anywhere but Missouri and the "easy states" he needs to win (NC, In, FL, Oh, Va, Co)? Romney wins if he gets these states. Obama wins one of these and he stays in....UNLESS: the next tier of battleground states, Ia, NH,Mn,Pa,Na,Wisc. The fight was always in these states, end of story. The fact that Obama didn't go into other republican won 2008 states means nothing.