My gosh, Rationalis, you appear not to grasp the inherent contradiction in your own words. In the same sentence you say "abortion is the killing of a ... PERSON" and then "the fetus is a POTENTIAL person." 

When you say that the fetus is only a "potential person," you grant to the "pro-abortionist" philosopher the very essence of their argument. A potential human is not yet a full human, it is a life growing, developing towards full humanity. Since the fetus is, according to you, not yet fully a person, then the secular moral agreement that we can not take the life of persons, does NOT apply to the fetus.

Au contraire, the Catholic position is that the fetus is a full human being in the most essential manner that makes humans, humans from the perspective of our theology. The fetus has been "ensouled" (to use an older terminology that has its own problems) and is thus immortal long before it is born. For us immortality is the essential distinguishing feature of humans versus animals.