Why the hell do you think the entire DNC went after his Senate seat with so much money?”
because they wanted a Demonocratic majority in the Senate.
And they rightly perceived Santorum to be vulnerable.
But there was another factor at play here. Santorum's senatorial opponent was Bob Casey. In Pennsylvania, the name Casey carries some magic, much like Kennedy in Massachusetts.
So here we had a vulnerable, uninspiring candidate (Santorum) against a magic name (Casey). Down went Santorum.
The same would happen if it were Santorum against the magic Obama. Except that Obama is a much, much better campaigner than Casey was.
Landslide Obama, fer sure.