I've actually traced this back to late 1970's and called this the "Southwest Swing." It tends to be socially conservative Democrats tending to vote for socially moderate to conservative Republicans.
A Republican perceived as a conservative will defeat a liberal Democrat in this area. For example, in 1992, Spector actually carried the region of every county bordering Allegheny and Washington Counties (inclusive), by just under 1,000 votes; he lost his home region badly. In 1986, he was 10-15 points ahead of William Scranton, III, who was perceived as an ex-hippie. Thornburgh in 1982 (possibly 1978 as well), Ridge in 1994, and Fisher in 2002 all saw a similar effect.
This effect has offset by the "Southeast Slump," which does just the opposite. As late as 1986, Scranton actually carried the southeast (all counties bordering a county bordering that boarders Delaware or Philadelphia Counties). GHW Bush might have 1988. Singel actually carried this region against Ridge in 1994.
In other words the states right is moving left, while the left is moving right.
Well put. The trend is overall static statewide with the east/southeast trending Democratic and the west trending GOP. Actually Allegheny IIRC is pretty static whereas the surrounding counties are trending heavily GOP. Philadelphia and to my dismay Bucks are pretty static, even micro-inching GOP. Difference is Montgomery, Delaware, and Chester are flying left and keeping the Dems alive here. My guess with Philly and Bucks is you have a similar phenomenon you have out west in lower Bucks county and Northeast Philadelphia being you have a lot of well-off union members who are socially conservative, but also pissed at the primarily African American Democratic run city government. I can sense a very slight "New Deal Die-off" as supersoulty called it here, but the extent of it is only seen in certain pockets of NE Philly and lower Bucks, and nowhere near what it is out west. One must also note the senior citizen population is pretty high in these areas.
