There used to be a time (in the not so distant past) when Republicans could count on a strong enough vote in the suburban counties surrounding Philly to partially off-set the numbers coming out of the city.
But now the suburbs are becoming almost as bad (politically) as the inner city is.
And that will hurt Republicans running in a state-wide race.
It's not so much registration trends as precincts that are still 2:1 Republican to Dem were voting for Kerry. Anger against the Bush Administration was running very high even back in 2004.
Suburban PA people still support Arlen Specter too. He is their idea of a “conservative” for balance.