Libertarians would seem like natural allies of the preborn. The fundamental principle from which libertarianism is deduced is non-aggression axiom. This axiom is simple: it is never right to commit aggression (violence) against another person, except in the case of self-defense. Common sense would seem to dictate, then, that abortion, the most grievous kind of aggression, would be opposed, but that is not the case for the majority of libertarians.
The Libertarian Party platform itself is pro-choice, saying the government has no place intervening between the woman and her womb's occupant. It's a private matter, they say, and a woman has the right to her own body, ignoring the claims of the preborn person to life, liberty and his/her own bodily integrity.
Some libertarians will go so far as to acknowledge that the fetus is indeed a human person with rights, but those rights do not trump the mother's right to her own body, just as no one has a right to someone's private property. Murray N. Rothbard advanced such a claim in his "Ethics of Liberty", saying that the mother doesn't have the right to kill her baby, just to "evict" the child from the property of her body. If the baby dies, then he/she dies, but the mom is not unjust or guilty of any wrong doing.
The most libertarian politician of all time (arguably) is Ron Paul, who is himself pro-life, after witnessing an abortion by C-section when in medical school in the 1960's. To those who hold that pro-choice is the correct libertarian viewpoint, Ron Paul argues that such incorrect first principles lead to disastrous consequences:
"A case in point is a young libertarian leader I have heard about. He supports the "right" of a woman to remove an unwanted child from her body (i.e., her property) by killing and then expelling him or her. Therefore, he has consistently concluded, any property owner has the right to kill anyone on his property, for any reason... Such conclusions should make libertarians question the premises from which they are drawn."
Look to the real cause of a problem pregnancy from a property rights view. The fetus is innocent because he/she did not cause their own life or the pregnancy to happen. And because this causal chain did not start with the baby, abortion is simply aggression against innocent preborn human persons. Libertarians view the human body as the first claim of property rights (hence the use of the euphemism "eviction" for abortion). And in the case of an unwanted pregnancy, both the mother and the baby have rights over the property of their own bodies, which are supposedly now in conflict. So let us look at the moral claims from the perspective of property rights.
There are three different scenarios of legitimate use of force against an aggressor. First, if someone were to trespass onto your property in a threatening manner, you would be justified in using force to repel the aggressor from trespassing on your property. In a second scenario, imagine you dragged a passer-by onto your property, you cannot then shoot them for being a trespasser. You were the aggressor against the passer-by, who is an innocent. It would be the passer-by who would have legitimate claims to repel you, even if you were both on your property. This is because the cause of the trespass is entirely your fault.
Now let us imagine a third scenario. A bully grabbed a passer-by and dragged them on to your property. Again, it is not the fault of the passer-by that the trespass occurred and so remains innocent and blameless in this situation. The bully is the aggressor and any injustice created by the passer-by's presence on your property is solely the blame of the bully. The solution to this trespass is to repel the bully, the attacker, not the passer-by.
In order to discover who is to blame in all three scenarios, we have to understand the chain of causality. If we declare that causality doesn't matter, only the fact that someone is on my property, then it would be open season on every passer-by! Then any property owner could grab someone they didn't like, drag them onto their property, and execute them.
In the case of pregnancy, the libertarian pro-choice advocate would like to have us believe that all unwanted pregnancies are akin to the first scenario: the preborn baby is the unjust aggressor to the property of the mother's body. This is a logical, scientific, philosophical and theological absurdity!
The fetus had no choice in the sexual union that brought him/her about, had no input as to who the father and mother ought to be or when and where conception should take place. The baby is the innocent passer-by in the other two scenarios.
In the second, the baby is the consequence of consensual sex, even if the pregnancy was the result of failed contraceptive use, because the mother still chose to enter into the only state of affairs that may result in a pregnancy. In the third scenario, the baby is the consequence of rape, where he/she did not ask for their dad- the bully- to be a vile and sinister person, did not ask to be procreated in such a circumstance, but is nonetheless there in the situation as an innocent. We all recognize it as an abuse of justice to punish an innocent for the crimes of another.
Concluding thoughts. The Supreme Court's Roe v. Wade decision legalized abortion according to the worst logic imaginable: we don't know if the preborn baby is really human life or not, therefore a mother has the right to destroy him/her. To those fence-sitters that think this is a good argument, just remember the words of the G.K. Chesterton who tells us that if you are going to bury a man, it is not enough that you think he's dead, or that he's probably not alive. You have to know that he's dead! We have to err on the side of life if there is a doubt as to the life of an individual.
The same is true with abortion: if we don't know the preborn child's status, then we ought to err on the side of life. A mother does not have a right to a dead baby. No one does.