This is not complicated. Abortion is the taking of a human life. A defenseless human life. It is a violation of the 6th Commandment, "You shall not commit murder". The child is not a blob, and he is not a piece of his mother's body. He is a small person, who needs his mother to survive.
Viability is not the issue. None of us are "viable" outside of our natural environments. If I held your head under water for a minute, you wouldn't be very viable. Your natural environment is a nitrogen/oxygen atmosphere, with certain temperature and air pressure requirements. A little baby's natural environment is the womb, and of course, he, like you, wouldn't survive outside of that environment.
The primary answer to this is spiritual. The gospel of Jesus Christ will change hearts and minds. That said, it is also right for us to work politically, to outlaw this slaughter of the innocents.
Please see my other blog post: The Sin of Abortion. Also, see this great video: 180 movie.