It has been a teaching of the Catholic Church for many centuries that the fact that an act is immoral is not itself a sufficient reason for acts of that kind to be punishable under the law. Therefore, before American Catholics or their bishops actively support laws making abortion illegal and punishable, they must carefully examine such laws to determine whether they are consistent with the common good and thus morally justifiable. This article first turns to the three conditions that Thomas Aquinas offers in the Treatise on Law (Summa Theologica, I-II, QQ. 90-105) that a law must fulfill in order for it to be consistent with the common good. Serious reasons are identified for doubting that laws prohibiting and punishing abortion can fulfill Aquinas’s three conditions. There are also serious reasons based on Catholic teaching, i.e., that we are always obliged to follow our conscience, for concluding that, contrary to the common good, many conscientious persons would be mistakenly punished by such laws. For these reasons, the article proposes that American Catholics and their bishops should reexamine their support for laws punishing abortion and should consider instead actively opposing such laws.