Actually your analysis is incorrect... PA is moving REDDER every election cycle. Pa has a large huge red T in the middle of the state, a huge blue death trap around philly and a blue sinkhole around Pittsburgh.
Now Pittsburgh region has been moving republican slowly for many cycles.. don't believe me, look at the county by county presidential elections in 2000 v 2004... Allegheny count (Pittsburgh Proper) may never go Republican, but the remainder of the 7-13 counties around Pittsburg are moving solidly into the Republican territory.
Philly is the last bastion of hard core liberal support in the state, and that is the part of the state where any republican must at LEAST make an ok showing if they want to win the state... they could get massive amounts of support from the rest of the state, but if they don't come out of Philly with at least some support they can't win.
PA is a red state, with a suburb of NYC in its eastern border dragging it down.