I think most reasonable people would say that a 9-month fetus, ready to be born, viable outside the mother's body is a human being with all the rights and privileges appertaining thereto. But the same reasonable people would say that a single-celled fertilized egg, invisible to the naked eye, with no organs, no brain, no consciousness, no personality, is NOT a person. The question is, and has always been: Where between these two extremes does life begin?
Before 1980 the huge majority of the pro-life movement was Roman Catholic. Protestants didn't care that much about abortion, it just wasn't that big of an issue to them. There was some uncertainty among Protestant Christians as to just when life begins. Roman Catholics believed it began at fertilization but remember Catholics are also against birth control.
The idea that life begins at conception came about for Protestants in 1980, and this was part of the Reagan presidential campaign. Life beginning when the sperm hits the egg, this is not a technical 'fact', it is not in the Bible, it is an arbitrary and artificial decision. It is a -political- decision, not a religious one.
This decision drove a stake in the ground for Fundamentalist Christians to rally around. When they began hearing this it became almost immediately a part of the religion.
The Republican Party, starting with Nixon and going on to Reagan, corrupted certain powerful spiritual leaders and got them to teach their flocks that it was their Christian duty to vote for one party over the other. The abortion issue was the keystone of this strategy to get Christians to vote Republican in 1980, and since.
We know today that at least half of all fertilized eggs never implant on the uterine wall and develop. Can you believe that God aborts more than half of all new 'human beings'? But because the idea of life beginning at fertilization is a -religious- idea, it is never questioned by good Christians.