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.