oBAMA only won by 5% over Romney.
So if Hillary loses 3% of her support and Trump gains 3% over what Romney got, then he wins. And early ballots in other states suggest democrat turnout will be way less probably much less than 3% from 2012.
Obama’s statewide plurality in 2012 was about 300,000. His plurality in Philadelphia was about 500,000. So Romney won the rest of the state outside Philadelphia. (Of course, not every county.)