This is politics. He has to try to appeal to the voters of Pennsylvania. Specter's problem is that he is duplicitous, he is a back-stabber, he is amoral, he is dishonest and uncompromising. The fact that he's liberal is almost an afterthought. Santorum is none of those things.
Santorum has a tough task ahead of him. Not only will the media's demonization of him hurt his chances with the 'moderate' suburbanites, but he will also have to work to keep those social and cultural conservatives who may otherwise favor Democrats on other issues, but who vote GOP because of that social conservatism.
Casey Jr is no doubt a phony, as what good does it do to be pro-life, or anti-gay marriage if your very presence in the senate (caucasing with Dems) increases the chances of horrible judges getting appointed, thus making both of those stated positions meaningless? He's already said he doesn't favor a Constitutional approach to fighting gay marriage, and that of course translates into support of letting judges have the final say. And I think he has sided with the Dems in the judicial filibuster fight.
If Santorum hopes to keep these social conservatives he must make all of this clear to them.
It will be interesting to see how the media treats Casey for his alleged holding of the same general beliefs as Santorum on these hot-button issues. It will be fascinating to see how they portray Santorum as an extremist pro-life, anti-gay marriage ideologue, while Casey as an enlightened pro-life, anti-gay marriage moderate.