The low-down on the Keystone State:
Broad context: PA outside of Philly County has been trending red for 20 years. It has so far been checked by Dem turnout in Philly County, but Philly County's population has been flat. So turnout increases in the county are from turnout machines/enthusiasm alone. At some point, that could breakdown.
(a) Total PA turnout is up 3% over 2008. Philly County comprises 11.5% of total PA electorate (similar to 2004, less than 2008).
(b) Romney wins non-Philly county 54-46. (Slightly better than Bush '04, who won 52.5 to 47.5)
Obama MUST net 433k votes out of Philly County to win. In 2008 he netted 478k votes. In 2004 Kerry netted 410k votes. In 2000 Gore netted 350k votes.
Tweak the assumptions to lower Philly turnout, increase non-Philly turnout, increase Romney share of non-Philly. And Keystone State goes red.