Pennsylvania's Democrats took a beating in 2010, losing five House seats, an open Senate seat and the governorship. You might think that they couldn't lose any additional seats in 2012 but they'll be three lawmakers that will be in great danger in the next cycle. Jason Altmire (CD4), Mark Critz (CD 12) and Tim Holden (CD 17).
Altimre's district is just west of Pittsburgh and is a bit red, having voted for Bush in 2004 and McCain in 2008. He won reelection with a weak 51% against an underfunded opponent and the GOP in the state legislature who control the redistricting process will ensure that more conservative neighborhoods are drawn into his district for the 2012 cycle.
Critz has one of the more gerrymandered districts in the country and its centered around Johnstown. This seat was drawn specifically to protect the late John Murtha but there won't be any reason to do it again now that Critz has taken over. Critz won by less than 2% of the vote and the district is surrounded by red areas that the stete legislature will be able to include for the next cycle.
Tim Holden won 56% of the vote in this rural east-central district. A true moderate, he's got a lot of support from all segments of the political spectrum which have kept him in power. There's no reason to believe that the GOP legislature won't drastically rewrite the districts lines and make his seat much redder for the 2012 cycle. Moreover, he'll have to introduce himself to a large part of this new district in a year that will probably be very tough for Pennsylvania Democrats.
The Keystone State will lose a seat in a congressional seat in the reapportionment and it will almost certainly be at the expense of one or more Democratic legislators. You'll see this scenario play out in quite a few states; Michigan, Ohio, North Carolina and Virginia-just to name a few.