I recently had a small argument with a Catholic friend of mine who kept approaching the abortion issue from the angle of Church teaching. However, the abortion issue falls under natural law. Abortion isn't wrong just because the Church teaches it's wrong. Abortion is wrong because it is an intrinsic evil, that is, it is the termination of an innocent human life. Murder, the killing of an innocent human being, is something that all persons intrinsically know is wrong. The stage of development does not make a person any less human or alive. Thus the Church's teachings on abortion affirm this natural order. Abortion is not a religious issue per se, it is a human rights and justice issue.
Senator Biden was acting like abortion was an issue similar to the Immaculate Conception. A Catholic can't make laws that say everyone has to believe in the Immaculate Conception. But Catholics as well as any human person are obliged to follow the natural law and work to stop the killing of innocent human lives which happen in abortion. This is also why abortion is never morally permissible.
The US Bishop's article follows [with my emphasis]
Bishops Respond To Senator Biden’s Statements Regarding Church Teaching On Abortion
WASHINGTON—Cardinal Justin F. Rigali, chairman of the U.S. Bishops’ Committee on Pro-Life Activities, and Bishop William E. Lori, chairman, U.S. Bishops Committee on Doctrine, issued the following statement:
Recently we had a duty to clarify the Catholic Church’s constant teaching against abortion, to correct misrepresentations of that teaching by House Speaker Nancy Pelosi on “Meet the Press” (see http://www.usccb.org/comm/archives/2008/08-120.shtml). On September 7, again on “Meet the Press,” Senator Joseph Biden made some statements about that teaching that also deserve a response.
Senator Biden did not claim that Catholic teaching allows or has ever allowed abortion. He said rightly that human life begins “at the moment of conception,” and that Catholics and others who recognize this should not be required by others to pay for abortions with their taxes.
However, the Senator’s claim that the beginning of human life is a “personal and private” matter of religious faith, one which cannot be “imposed” on others, does not reflect the truth of the matter. The Church recognizes that the obligation to protect unborn human life rests on the answer to two questions, neither of which is private or specifically religious.
The first is a biological question: When does a new human life begin? When is there a new living organism of the human species, distinct from mother and father and ready to develop and mature if given a nurturing environment? While ancient thinkers had little verifiable knowledge to help them answer this question, today embryology textbooks confirm that a new human life begins at conception (see www.usccb.org/prolife/issues/bioethic/fact298.shtml). The Catholic Church does not teach this as a matter of faith; it acknowledges it as a matter of objective fact.
The second is a moral question, with legal and political consequences: Which living members of the human species should be seen as having fundamental human rights, such as a right not to be killed? The Catholic Church’s answer is: Everybody. No human being should be treated as lacking human rights, and we have no business dividing humanity into those who are valuable enough to warrant protection and those who are not. This is not solely a Catholic teaching, but a principle of natural law accessible to all people of good will. The framers of the Declaration of Independence pointed to the same basic truth by speaking of inalienable rights, bestowed on all members of the human race not by any human power, but by their Creator. Those who hold a narrower and more exclusionary view have the burden of explaining why we should divide humanity into those who have moral value and those who do not and why their particular choice of where to draw that line can be sustained in a pluralistic society. Such views pose a serious threat to the dignity and rights of other poor and vulnerable members of the human family who need and deserve our respect and protection.
While in past centuries biological knowledge was often inaccurate, modern science leaves no excuse for anyone to deny the humanity of the unborn child. Protection of innocent human life is not an imposition of personal religious conviction but a demand of justice.