Yes, that's the only way. Casey, Jr. is the ultimate "invisible man". He toes the party line but is never seen out there championing it.
The GOP challenger will have to be aggressive in tying Obama around Casey's neck and harp on how Obama/Casey's policies have hurt PA.
Casey is lucky that his re-election is falling in a presidential election year because he'll benefit from increased Democrat turnout in urban areas. If the Toomey/Sestak matchup had occurred in a presidential year, I think Toomey would probably have lost.
I do think that Jake Corman has potential, provided that he can raise the necessary money. Though he comes from central Pennsylvania, he may be seen as a western Pennsylvanian (traditionally, Pennsylvania elects one U.S. Senator from eastern PA and one from western PA, and the other Senator is from the eastern part of the state). Though from Scranton, Casey has been seen by many in Western PA as one of their own. Corman could benefit from the support of Santorum’s following, having once served on his staff.