Question about abortion and the Bible

So I read a comment elsewhere (local paper) that all Christians must vote against abortion rights. I replied that Not only do many Christians support a woman's right to choose, the Bible never mentions abortion. A reply referred to Psalms 139:13-16 as a reference to abortion. So I dragged out my Bibles and read various translations. I saw no reference to abortion. Further, what I read could be interpreted to mean that an unformed, imperfect substance in the womb is not yet a person with a soul. It certainly isn't an unequivocal pronouncement on abortion, or a commentary on the subject at all.

If you oppose abortion, that's certainly your right, but I don't see any way one could claim that the Bible speaks in any way on the subject.

SO, I guess my question boils down to asking for opinions. In your opinion, is opposition to abortion a necessary requirement for being a Christian?