We sometimes act like we care more about persuading people that abortion is wrong than we care about persuading people that Jesus is Lord. In fact, we disavow the latter in the course of arguing for the former.
It won't work ultimately. Natural law arguments simply aren't that convincing when separated from the life of virtue--from the Christian way of life (or, at the very least, something in the classical stream of Western and "Judeo-Christian" way of life). Moreover, it concedes secularism's premise that a just society and a moral life is possible without God and his revelation, when, in fact, the evidence is all around us that this isn't so.
That doesn't mean we shouldn't make natural law arguments. We should make them, but it's not something separate from evangelization. The consistency and truth of the Church's moral teaching--i.e., the goodness of the Christian way of life--gives witness to the truth of the Gospel. Convincing someone that abortion is wrong is part of bringing them closer to Christ.
