In his 1989 essay Why Abortion is Immoral, Don Marquis notes that “a sketch of standard anti-abortion and pro-choice arguments exhibits how those arguments 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 Defending Life: A Moral and Legal Case Against Abortion Choice, which Marquis does not address.
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 itself attempted to take a neutral or agnostic position toward the personhood of the unborn. And I myself find it hard from time to time to feel that the unborn (at least in their pre-sentient state) are the subjects of rights that I know they are. Perhaps this is true of you. This means that we ought to then present the best arguments for our case to persuade the public that unborn humans are persons.
However, the persuasion of others to affirming the personhood of the unborn is not the chief purpose of the pro-life movement. The preservation of human life is. Widespread conversion to our way of thinking is simply a means to that end. Until everyone agrees, we can still undermine abortion by appealing to their agnosticism.
If a person is hesitant to affirm that the unborn are persons, we can ask: “What if the arguments are evenly or sufficiently matched? Should abortion be allowed?” Then we can argue, as Beckwith does, 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'.” Abortion would be comparable to detonating a building where there is a chance that person is in the building. Do we detonate since the odds are against there being a person in there, or it is not certain that I’d be destroying a person? Or do we abstain from destroying it as long as we don’t know? I’m sure that no one would destroy the building even if there was just a 1 in 100 chance of there being a person in it. Thus, since in the minds of the undecided, the chance that the unborn are subjects of rights is presumably greater than 1 in 100 (probably closer to 1 in 2), they should conclude that abortion is prima facie morally wrong. If personhood is in doubt, so is the right to elective abortion, and thus to excerize or permit its exercise would be reckless. (See Defending Life, pp. 149 – 152; author Francis Beckwith also addresses the pro-choice appeal to agnosticism.)
By doing this we can begin to turn people’s opinion against elective abortion, even if their view of the unborn themselves doesn't reform until later. The appeal to agnosticism is only the start, a start that will hopefully save lives, to presenting the pro-life case in its entirety.