May a God-fearing legislator vote for an abortion law that excepts cases of rape, incest, or danger to life of the mother? "Never!" snap some of our best pro-life people, sometimes with a tincture of impatience and shrillness in their voice.
At ease! The ancient wisdom of moral discernment distinguishes between positive obligations and negative prohibitions. Positive obligations, on the one hand, bind us to do whatever is possible while leaving the rest to God. Positive laws have upper limits and admit exceptions. On the other hand, negative obligations forbid us from ever doing anything wicked. Negative laws know no upper limits and do not admit exceptions. Martyrs are keenly aware of the difference.
The Catechism of the Catholic Church proposes the civic duty to penalize abortions as a positive obligation, not as a negative prohibition.
2273 ...As a consequence of the respect and protection which must be ensured for the unborn child from the moment of conception, the law must provide appropriate penal sanctions for every deliberate violation of the child's rights.
That is evidently a positive obligation, not a negative prohibition. In clipped Latin such a law is described as "lex positiva valet semper sed non pro semper". That is, a positive law binds universally but its application is flexible. "Thou shalt keep holy the Sabbath Day" is an example. If you are at the South Pole you may be excused from attending Sunday Mass. Another example is: "Thou shalt love thy neighbor as thyself." Give alms to the needy, but don't sell your house and farm to do that.
A negative law, to the contrary, admits no exceptions: "lex negativa valet semper et pro semper." Under no circumstances whatsoever may one break the law. Martyrs lay their heads on the block rather than adore false gods. St. Lawrence, being roasted on the gridiron, joked to the executioners to turn him over now that his one side was already well done. Saint Cyprian told the judge to not waste more words because he was not about to change his mind. He stood ready for the sword, and then his head fell. Saints Perpetua and Felicity exposed themselves to the sword even as their babies cried besides them. Negative laws test the mettle of martyrs.
"Thou shalt not kill an innocent person" is a negative law. It obliges always and in every case, admitting no exceptions. The Magisterium teaches that what is in itself wicked can never be made good by attempting to offer it to God:
The negative moral precepts, which declare that the choice of certain actions is morally unacceptable, have an absolute value for human freedom: they are valid always and everywhere, without exception. They make it clear that the choice of certain ways of acting is radically incompatible with the love of God and with the dignity of the person created in his image. Such choices cannot be redeemed by the goodness of any intention or of any consequence; they are irrevocably opposed to the bond between persons; they contradict the fundamental decision to direct one's life to God" (Encyclical Evangelium Vitae, No. 75),
What happens, then, if a positive civil law contradicts a negative divine law? Nothing happens. Let us suppose that a civil law would state: "It is hereby permitted to abort children conceived by rape or incest or who endanger the life of the mother." The legislation is words without meaning. It is a fraud, like a counterfeit dollar bill. It should deceive no one, not even eager radical feminists.
God's law "Thou shalt not kill" protects the inviolable right of an innocent unborn child to life. This remains permanently true even when Supreme Justices in awesome uniform pretend to not know that, or when manipulators of the media cackle the opposite, or when hallowed UN officials stupidly kill babies to prevent overpopulation. Abortion is and remains an abominable crime, all shouting to the contrary notwithstanding. Catholics who procure an abortion are excommunicated from the Church: "A person who actually procures an abortion incurs a lata sententia excommunication" (Canon 1398). The words lata sententia mean "automatically," without need of action by the lawgiver.
But what happens when a government withholds punitive measures from abortions in case of rape, incest, or life of the mother? It means simply that the state withholds penalties in the case of those exceptions. This is normally illegitimate, but under defined circumstances it may be a legitimate and proper course for legislators to follow, as the Church teaches:
A particular problem of conscience can arise in cases where a legislative vote would be decisive for the passage of a more restrictive law, aimed at limiting the number of authorized abortions, in place of a more permissive law already passed or ready to be voted on.... An elected official, whose absolute personal opposition to procured abortion was well known, could licitly support proposals aimed at limiting the harm done by such a law ... This does not in fact represent an illicit cooperation with an unjust law, but rather a legitimate and proper attempt to limit its evil aspects (Evangelium Vitae 73).
In fact, Poland enacted a strict abortion law that allows some exceptions. The results are heartening. That law collapsed the number of legal abortions per year from 150,000 to 138 (see International Right to Life Newsletter, XIV,1) That is a whopping decrease, down to one-one thousandth of what it was before the imperfect law went into effect. It is not yet 100% closure, but it is close. It disproves arguments proclaimed sonorously by legal purists that laws with exceptions are ineffective. Arguments against the fact of Poland hold no water. So, let incremental restrictions win the day in America too. Be ye wise as serpents and simple as doves.