Benedictine Father Paul Marx is credited with seeing the link between contraception and abortion early in his pro-life career (see story, this page). Pope John Paul II explains in his encyclical Evangelium Vitae (The Gospel of Life) that contraception doesn't prevent abortion from becoming widespread — it encourages it.

It is frequently asserted that contraception, if made safe and available to all, is the most effective remedy against abortion. The Catholic Church is then accused of actually promoting abortion, because she obstinately continues to teach the moral unlawfulness of contraception. When looked at carefully, this objection is clearly unfounded. It may be that many people use contraception with a view to excluding the subsequent temptation of abortion.

But the negative values inherent in the “contraceptive mentality” — which is very different from responsible parenthood, lived in respect for the full truth of the conjugal act — are such that they in fact strengthen this temptation when an unwanted life is conceived. Indeed, the pro-abortion culture is especially strong precisely where the Church's teaching on contraception is rejected (No. 13).