There's a strong difference in the two things. Edwards as VP candidate with Kerry did not bring in his home state of NC. That has happened to lots of VP candidates and isn't a big deal. But Santorum was tossed from his Senate seat by his home state. You notice Edwards left the Senate so he would not have to suffer the ignomy of losing a Senate reelection. That type of defeat IS a big deal. As for Santorum for VP, I would not completely rule that out. But it would be kind of "iffy", I think...