The Census Cometh, and the Census Taketh Away
The Pennsylvania General Assembly has 203 seats and like congressional districts will have to be redistricted following the 2010 census. Slow but increasing population in Pennsylvania coupled with slowly decreasing population in Allegheny County means that the county's state delegation is going to shrink. The only question is by how much? What that math looks like to me:
Pennsylvania's Population: 12,299,533
Average Pennsylvania House District: 60,589 people
Allegheny County: 1,279,914
So if you do the division you had 21.1 house seats in the county.
For 2009 Pennsylvania's population is estimated at: 12,604,767. If that is correct and the trend continues one more year gives an estimated population in 2010 of 12,638,682. Which then would mean that each of the 203 districts of the Pennsylvania House should measure near 62,260.
Allegheny County's 2008 population estimate is 1,215,103. Again, if that is correct and the trend continues through until 2010 it gives a projected population of 1,198,900. Divide that by the projected district size of 62,260 and you 19.3 districts.
So Allegheny County itself is losing not just one, but 1.8, virtually 2 whole, districts in the Pennsylvania General Assembly. Out of 20 that is a lot. Which districts are the most 'vulnerable'. First, what does vulnerable mean? Each and every district is likely to see significant boundary changes. 'Saving' a district usually means keeping an incumbent's residence of record within a district that roughly keeps his or her political base.
How it all plays out is really quite impossible to speculate on too precisely at this point. Redistricting is about as political a process as exists. Some have said that the 20th district vacated by Don Walko is now tops on the vulnerability list. That's possible, though I'm not sure. The argument is that the most junior incumbents will be most likely to 'lose' their seats. It's one argument, but it isn't the only thing going on. The party that controls the process may choose to target a particular district or a particular incumbent. That and there could be a retirement or two among the incumbents over the course of the next two years... which would mean PA20's incumbent would not be the most junior when redistricting actually happens.