It doesn't have to be murder to be wrong. I believe that premarital sex is wrong. And not taking responsibility for the results of one's wrong choices is wrong. But if the pregnancy was not the result of the mother's wrong choices then abortion is not failing to take responsibility so it wouldn't necessarily be wrong on those grounds.
But then why is pre-marital sex wrong?
Maybe this is th atheist point of view, but I see right and wrong not based on a book of rules written 3000 years ago, where we ignore most of those rules, anyway, but on a more pragmatic view.
Something is right or wrong if it harms another person. The fundemental question here is a fetus a person, not whether it feels pain or not. (which, by the way, is a matter of debate when a fetus actually can feel pain.)
But I'm not trying to impose my beliefs on someone else. And that's a luxury I can afford since I don't believe abortion to be murder. (If I did believe it to be murder then I would probably feel obligated to fight to get it legally recognized as murder.)
I'm not happy with people shredding or poisoning or otherwise causing pain to the tiny body once it has reached the point where it can feel pain, but as a general and practical guide it makes good sense to me to use fetal viability to help draw the line of when the fetus starts to have its own claim to protections under the law.
But those laws are not well written, which is why this is up for debate, I think.
Now, Santorum, good guy. I think his view is wrong on this, but he strikes me as sincere and not checking with a focus group to see what the best answer is.