Today Jay Cost of RealClearPolitics conducts another of his impressive county-level analyses of a swing state, in this case Pennsylvania.

His bottom line is that the Keystone State has maintained a very consistent Democratic advantage (compared to the national vote) of about 4 percentage points over the last four decades, despite internal shifts. Basically, growth of the Democratic vote percentage in Philadelphia and its closer suburbs has been offset by a Republican trend in western PA, and the Philly exurbs. Cost identifies northeast PA (where Obama did very poorly in the primary) as an opportunity area for McCain, and the Philly exurbs as one for Obama.

Cost does not get into another X-factor: the astonishing pro-Democratic trend in voter registration in PA since 2004, a net shift of 486,000 votes, according to a recent study by Rhodes Cook. Ds now enjoy a party-registration plurality in the state of more than a million votes.

All in all, it's hard to imagine Obama losing PA if he's running even nationally.