This is the central issue in the social teaching of the Church; the one that has forever been considered the gravest of sins, and though there is a continual urge by many Catholics, either from a lack of knowledge or design, to reduce it in importance or conflate it with other social teaching issues, it remains alone in its paramount importance.
Cardinal Ratzinger, before becoming Pope Benedict XVI wrote about this.
Here is an excerpt.
“One widespread section of public position in the educated bourgeoisie may find it exaggerated and inopportune--indeed, downright distasteful--that we continue to remind them that the problem of respect for a life that has been conceived and is not yet born is a decisive question.
“In the last fifteen years, almost all Western countries have legalized abortion, to the accompaniment of lacerating debates; ought we not today to consider the problem settled and avoid brushing the dust off antagonistic ideological positions that have been made obsolete by the course of events?
“Why not accept that we have lost the battle and choose instead to dedicate our energies to initiatives that can hope to find support in a broader social consensus?
“Indeed, if we remain on the superficial level, we could be convinced that the legal approval of abortion has not really changed much in our private lives and in the life of our societies; basically, everything seems to be going on as before.
“Everyone can act in accordance with his conscience: a woman who does not want to have an abortion is not compelled to do so, and a woman who does have an abortion with the approval of a law would perhaps have done so in any case (or so we are told).
“It all takes place in the silence of an operating room, which at least guarantees that the "medical intervention" will take place with a certain degree of safety: and it is as if the fetus that will never se the light of day in fact never existed.
“Who notices what's going on? Why should we continue to speak publicly of this drama? Is it not perhaps better to leave it buried in the silence of the consciences of the individuals involved?
“The Book of Genesis contains a passage that addresses our problem with impressive eloquence: the blessing the Lord God pronounces on Noah and his sons after the flood. After the event of sin, God reestablishes here, once and for all, the only laws that can guarantee the continuation of life for the human race…
“The text from Genesis guides our reflections in a double, which corresponds well to the double dimension of the questions we asked at the beginning of this essay:
“First, there are no "small murders". The respect of every human life is an essential condition if a societal worthy of the name is to be possible.
“Secondly, when man's conscience loses respect for life as something sacred, he inevitably ends by losing his own identity.”