In his 1989 essay Why Abortion is Immoral, Don Marquis notes that the arguments made by people on both sides of the abortion debate “possess certain symmetries that explain why partisans of those positions are so convinced of the correctness of their own positions, why they are not successful in convincing their opponents, and why, to others, this issue seems to be unresolvable.” We may concede as much about popular-level arguments, though I think the more sophisticated pro-life arguments are significantly better than their pro-choice counterparts; I particularly recommend the discussion of the “substance view,” as described in Francis J. Beckwith’s Defending Life: A Moral and Legal Case against Abortion Choice, and in Patrick Lee's Abortion and Unborn Human Life (second edition), among others. (Marquis does not address this view in his essay.)
Regardless, I can see why someone who is undecided on abortion or the question of the unborn’s personhood can feel that such questions are unresolvable. The Supreme Court of the United States took the position that the debate over the unborn’s status was a stalemate and attempted to take a neutral or agnostic position toward the personhood of the unborn. All of this might lead us to think that convincing others is impossible, and our motivation to learn, present, or refine the best arguments available might diminish. Nevertheless, we must present our case that unborn humans are persons.
However, persuading others to affirm the personhood of the unborn is not the only way to preserve intrinsically valuable human life, one of the pro-life movement’s central aims. Widespread acceptance of our view on the reality of the unborn as persons is, of course, a powerful means to that end. However, we can also undermine abortion by appealing to the agnosticism of many pro-choicers, those who view each side's case as equally plausible, yet affirm a right to abortion nonetheless.
If we're talking to a pro-choicer who argues that abortion should be legal, because both sides make compelling arguments, we could respond by saying, "Wouldn't the presence of compelling arguments on both sides actually support pro-life laws banning abortion?" Beckwith argues that “if one is not sure that one is killing a moral subject, then one should not kill it. That is the unborn should be given 'the benefit of the doubt'.” Why? Keep in mind that if each side makes compelling arguments, then either outcome of the controversy, if enshrined in law—the killing millions upon millions of unborn human beings or the constraining of a woman's freedom by a prohibition on relieving herself of the substantial burdens of pregnancy and raising her child post-birth—is just as likely as the other to be evil. Faced with these two possible outcomes, we must ask the pro-choicer, which is the worse outcome? I think the former certainly is.
Why assume that human life (and not liberty or choice) should be given the benefit of the doubt here? Because, given such agnosticism on the personhood of the unborn and the permissibility of abortion, performing an abortion would be comparable to being paid to blow up a building where there is, say, a 40 or 50-percent chance that a nine-year old is in the building. Do we detonate, since there might not be a person in there? Or do we abstain from destroying the building, since, for all we do know, there might be a person in there? What if we desperately need the money being offered to blow up the building, to get out of debt, or obtain medical services, or move out of a crime-ridden area of town (for the good of our family)? Even then, I’m sure that no one would destroy the building if there were a 40 or 50-percent chance that the nine-year old was in it, Or, if someone did destroy the building, we would judge that he had acted impermissibly. It is a basic moral principle that it is better to suffer evil than to deal it to others.
In the minds of the undecided, the chance that the unborn are subjects of rights, which abortion unjustly violates, is comparable to the chance of there being a nine-year old in that building. Furthermore, as far as the undecided are concerned, abortion is quite like that act of destroying the building. Thus, they should conclude that abortion is prima facie morally wrong. If the unborn's personhood is in doubt to such a considerable degree, so is the right to elective abortion. To exercise such a supposed right to abortion, or to permit its exercise would be a reckless disregard for intrinsically valuable human life—even if it can be shown later that the unborn were never persons.
By taking this approach we can begin to turn people’s opinions against elective abortion, even if their view of the unborn doesn't change until later. This approach is only the start to presenting your pro-life case in its entirety, but it is one that will spur a greater interest in considering the issues involved and will save lives.