Here's teh thing. If it was something like Casey 42 Santorum 35, you could argue Santorum wasn't in too much trouble. But Casey's almost at 50 this early. Santorum has a lot of ground to cover, he's going to need almost all the undecideds (since they break against the incumbent usually not easily) and going to sway a few Casey voters back. Anything can happen in a campaign, but having your opponent at almost 50 this early is not good.
The way I see it, the Casey name almost makes him the incumbent.
Lets put it this way...I think more people recognize Casey than Santorum...hell a very well educated, fairly liberal friend of mine confused Santorum with Specter...he certainly knows who Casey is.
Look at how Democrats have won and lost this state in the last 20 or so years....
Carter=fairly populist...lost in 80
Mondale=populist lost in 84
Dukakis=came close to winning in 88, lost
Wofford won a special election...but I chalk that up to no one wanting 3 different senators in a short time span
Clinton-Gore-Kerry-won 92-04
You guys haven't won a senate seat here (in a regular election) in God knows how long...
when you guys run libertarian like centrists (socially liberal, moderate on fiscal issues, Clinton, Rendell) you win...when you run liberals...you come close, or win (Kerry, Gore).
When you run populists you generally lose.
Now think of it this way
Conservative Democrats (likely the catholic target group) have a choice...a Republican (albeit conservative on fiscal issues) who has a lot of clout and is pro life...vs a Democrat who is prolife and is likely to be a backbencher [to borrow the UK term] and marginalized like his father was nationally (casey should run for governor, he can't be marginalized there)...
If you're voting on social issues, let alone abortion...who's the better choice?