George Bush has wiggled himself out of his most recent position on abortion and into a new one. The vice president now favors its legality when pregnancy results from rape or incest as well as when the mother's life is in danger. For Bush, the first two exceptions are new, the last one old. Where once he stood united with President Reagan on the issue, now he is most tenuously tethered. Bush must find it scary being his own man.

The latest vice-presidential pronouncement on abortion produces, like the issue itself, some contradictory feelings. First comes the urge to ask where he's been all these years and why he had felt it necessary on an issue of conscience to parrot Reagan's political line. After all, only three years ago Bush, in his usual rah-rah fashion, cheered, ''My position is like Ronald Reagan's. Put that down, mark that down.''

That position, having been both marked and put down, now turns out to be adrift. Where once Bush said his thinking had undergone an ''evolution'' toward Reagan's own, it now seems to be evolving back to where it started. That, of course, is good news, since it's always heartening to see a smart man act independently and not as an intellectual eunuch in the court of Ronald Reagan.

In fact, the more strident the vice president was on the issue of abortion, the more suspect he became. To paraphrase Gertrude's remark to her son Hamlet, Bush protested too much.

But now Bush has gone from the unbelievable to the untenable. Unlike Reagan, who would permit abortion only when the mother's life is in danger, the vice president has now sunk into the muck of moral relativism where the well-being of the woman is taken into account. After all, a fetus conceived by either rape or incest is still, in the vernacular of the right-to-life movement, an unborn life. It is hardly its fault that it was conceived criminally, maybe violently, and certainly immorally. No matter what, it remains what Bush says he cherishes above all things: a human life.

The quicksand into which Bush has stepped is a crowded bog. It holds those of us who are morally repulsed by the casual taking of either (choose your terms) potential life or life itself, but see no alternative to granting women the right to make these decisions for themselves. The sympathy Bush extends to the pregnant rape victim is understandable. But does the woman whose child will be born deformed and, ultimately, doomed deserve any less sympathy?

And how about a teen-age girl, pregnant out of impulse and ignorance? Is her plight -- pregnant at 13, a mother at 14 -- significantly less tragic than that of a mature woman who conceives as a result of rape? Is that girl, walking through the hallways of her school, more at ease with her plight, more comfortable with her prospects, than a rape victim? The answer -- the only answer -- is: Who knows? And since neither you nor I can know, what is our choice but to leave the decision to the women involved?

In Iowa, where Bush promulgated his new position on abortion, he movingly recounted the christening last year of his adopted granddaughter: ''My wife and I stood there and watched and thanked God . . . her mother . . . came down on the side of life.'' Bush's emotions are evidently sincere, but his logic is faulty. The child born of an incestuous relationship or the one produced by rape would be just as worthy of adoption. He, too, would be life. It seems it's not the issue of life that moves Bush to punch two holes into his anti-abortion stance -- it's the plight of the mother.

Fine. The plight of the mother -- and not just of the child -- also concerns the pro-choice movement. But what's troubling about Bush's position is the suggestion that abortion is only permissible when a woman has clearly been victimized -- when she's not an accomplice to her plight.

Rape clearly satisfies that requirement and, usually, so does incest. And, of course, it's no one's fault if a choice has to be made between lives, the woman's or the fetus'. That, in turn, suggests a punitive role for pregnancy and childbirth -- the woman getting what she deserves. This has precious little to do with the sanctity of life and lots to do with moral judgments about sexuality.

But either abortion is murder or it is not. For years, Bush has been saying it is, echoing Reagan's position. If that's the case, then there can be no exception for fetuses produced by incest or rape.

George Bush, caught once again between his personal values and his political ambitions, has sprung a trap on himself. His muddle comes from wanting to be president more than he wants to be right. The way he's going, he'll be neither.