Sen. Sam Nunn (D-Ga.) has introduced a new line of argument to sustain his tergiversation on the matter of abortions. He has been adamant in the past in opposing choice. He even advocated at one point a constitutional amendment, as required, to overturn Roe v. Wade. On Sept. 4 he changed his position, and he gave as the reason for doing so the unenforceability of any comprehensive antiabortion law.

"I do not believe that large segments of the American people and the medical profession will accept or obey a law making early term abortions a criminal act after all those years," he said in a letter to his constituents. He sought to reiterate the moral point by adding, "I hope we have not reached the point in this country when anything that is not prohibited by law is considered morally and ethically acceptable."

The departure of Nunn from the pro-life legal ranks is a pretty heavy blow, since Nunn has for several years been thought of as the sober alternative to such as Mario Cuomo and Jesse Jackson as a Democratic standard bearer in 1992. His office hastened to say that nothing had altered the senator's insistence that he has "no plans at this time to seek the presidency." This may be true, and it may not be true. Most politicians would like to become president, and many of them adjust their politics in such a way as to minimize the difficulties they would face.

Nunn is a very bright man and must have known, when he suddenly took that fierce turn last year against the broader interpretation of the ABM Treaty, that his position would be greatly appealing to the pacifist types who insisted that testing in space was forbidden by the treaty. The collapse of the Soviet Union has all but undermined the argument about testing, and the Star Wars program goes somnolently along without anybody paying it much heed. It is no longer a trip-wire policy argument that sharply divides left and right. Abortion is.

Having said all of this, we should devote attention to the arguments used by Nunn. They are quite the opposite of the traditional choice arguments. Here the abortion lobby has talked about the dreadful biological and medical consequences of a repeal of Roe v. Wade. Sen. Edward Kennedy (D-Mass.), with his gift for the populist formulation, denounced Judge Robert Bork before Bork reached the Senate chamber by shouting out that if Bork were to go to the Supreme Court, American women would have to return to the coat hanger to get an abortion.

Suddenly we have a prominent politician who changes sides and says that if Roe v. Wade were reversed, and if the individual states were then to prohibit abortions, as they did before 1973, we would end up with an unenforceable law. Clearly he was not talking about coat-hanger abortions but about the clinical kind, performed in air-conditioned quarters under strict medical supervision. He seems to be saying that such abortions would continue in such profusion that it would be unlikely that the states would enforce the law.

Now I think that is a very interesting argument and more nearly convincing than not. We know that the antidrug laws are not enforced, except eccentrically: smoking marijuana in the streets is almost as safe as mugging someone. Now granted, dope-taking is something that can be done by anyone who can carry something the size of a cigarette pack, whereas the hardware required by an abortion clinic uses up trailer-size space.

But Nunn seems to be saying that if the abortion laws were to change, the probability is that they would not be enforced, never mind that they would need to be flouted in pretty conspicuous antilegal pillboxes. Houses of prostitution, which flourish in many cities, are relatively discreet, certainly by comparison with abortion centers. But Nunn's point survives: if abortion, which last year arrested the lives of just over 1.5 million people, is going to proceed anyway, there isn't much point in striving to make it illegal.

He closed his statement by saying that under the circumstances there was no alternative than to leave the decision whether to abort "to the informed conscience of the mother." But that is evasive thinking. An alternative is to leave the decision to the uninformed conscience of the mother. Nunn believes that abortion is the taking of innocent life. How can a conscience be "informed" if it reaches a contrary conclusion? He would have spoken with greater philosophical precision if he had said that the decision would have to be left to the mother, even if her conscience was undeveloped.

But then to have put it that way might not have suited his political aims.