The heart of Santorum's campaign is his unshakeable pro-life values. How could he endorse Trump, knowing that Trump was pro-partial-birth-abortion in 1999 (and possibly later), even while professing his "hatred" for the "concept of abortion." Whatever that means.
Btw, Trump's dodge on abortion is exactly the same as was taken by the late Mario Cuomo, liberal governor of NY.
Yeah, Trump has recently claimed that he changed his mind -- but he has never, to my knowledge, completely repudiated the stance he took in 1999. And he has never clarified his alleged new position.
Seriously? Santorum endorsed Arlen Specter.