WE ARE LONG PAST the time when all scientific progress was thought to be good. Nuclear bombs are the most dramatic example of science gone bad, but there are plenty of others.

Three Mile Island and Chernobyl proved what we already knew: that nuclear power is just too dangerous. Pesticides boost crop yields, but they also poison the land and, in some cases, our food. The internal-combustion engine contributes to global warming and the depletion of the ozone layer. Antibiotics were one of the great boons of the 20th century, but their overuse is giving rise to exotic, drug-resistant, 21st-century variants of diseases long thought to have been conquered, such as tuberculosis.

So it is no surprise that we are on the verge of outlawing human cloning. Having already absorbed the lesson that science is too important to leave to the scientists, we are now engaged in a national debate over whether to allow this most science-fictionish of medical technologies to proceed.

But even though the impulse is understandable, an outright ban on cloning would also be an enormous mistake. No one (well, hardly anyone) wants to see cloning used to create exact genetic replicas of people — or, once genetic engineering has advanced, to create enhanced superhumans with traits such as superior intelligence, resistance to disease, or, more controversially, a predisposition to heterosexuality. But research into embryonic-cloning technology, aimed at curing such illnesses and conditions as Parkinson’s disease, diabetes, and spinal-cord paralysis, is another matter altogether.

Last year, the US House of Representatives overwhelmingly passed a bill to ban all cloning — not just cloning aimed at creating new human beings, but also so-called therapeutic cloning, which involves the creation of embryos for the purpose of treating disease. Earlier this year, President Bush announced that he, too, supports a total ban on cloning.

Now the Senate is considering two bills. One, proposed by Sam Brownback, a Kansas Republican, and Mary Landrieu, a Louisiana Democrat, would emulate the House bill by banning all cloning. The other, sponsored by Dianne Feinstein, a California Democrat, and Arlen Specter, a Pennsylvania Republican, would outlaw reproductive cloning — that is, the kind aimed at manufacturing new people — but would allow therapeutic cloning. Significantly, Feinstein-Specter’s co-sponsors include not just liberal Democrats such as Massachusetts’s Ted Kennedy and New York’s Hillary Rodham Clinton, but also pro-life Republicans such as Utah’s Orrin Hatch.

The 100 senators, who are reported to be evenly divided, could vote on the measures before Memorial Day. A vote for Feinstein-Specter would allow promising medical research to continue. A vote for Brownback-Landrieu — which would almost certainly become law, given the positions already taken by the House and the president — would criminalize both types of cloning, thus halting scientific research that could help us lead longer, healthier lives.

Long the stuff of speculation in science fiction and films such as 1978’s The Boys from Brazil (in which Josef Mengele attempts to manufacture Hitler clones) and the latest Star Wars installment (titled Attack of the Clones), cloning became a reality in 1997, when a British scientist succeeded in creating a cloned sheep named "Dolly." That, in turn, sparked the current debate over humans.

The first step in creating a clone consists of removing the genetic nucleus from an egg — a sheep’s egg, a cow’s egg, or a human woman’s egg. The egg is then injected with the nucleus of a cell from another animal or person. Typically, skin cells are used. Once the transplanted nucleus is in place, the egg is given a jolt of electricity (a nice Frankensteinian touch), which — for reasons that are not entirely clear — begins to divide and develop, just as if it had been fertilized by a sperm cell. (Or, more likely, fails to divide and develop, since success in cloning is currently a rare thing.)

Normally a fertilized egg is the genetic progeny of two individuals, a male and a female, with half the genes coming from each partner to create an utterly unique being. A clone, though, receives all its genes from one source — the nucleus of a skin cell, to continue our example.

In reproductive cloning, the dividing, developing clone would be implanted in a woman’s uterus to grow into a fetus and, ultimately, a baby. The baby would be an exact genetic replica of the skin-cell donor — an identical twin, albeit of a completely different age. Clones could even be created from long-dead relatives. But in therapeutic cloning, division would be allowed to continue only for a few days, until the embryo had reached the "blastocyst" stage — somewhere between 100 and 200 cells. At that point, stem cells — cells that can be prodded into becoming any type of cell, be it heart, kidney, or brain — would be extracted, thus killing the embryo.

Last year, President Bush announced that he would allow federally funded stem-cell research to continue so long as scientists restricted their work to existing supplies of stem cells — as opposed to taking any new such cells from unwanted embryos stored at fertility clinics, even though those embryos were destined to be destroyed (see "Cellular Division," News and Features, July 13, 2001). Cloning extends and complicates the stem-cell issue. Because a clone is an exact genetic duplicate of the donor, its stem cells could be used to develop healthy new tissues that could be put back into the donor’s body without fear of rejection.

"If this research is allowed to succeed, by the time we grow old, this will be a routine thing," says Robert Lanza, of the Worcester-based Advanced Cell Technology, in the current Atlantic Monthly. "You’ll just go and get a skin cell removed at the doctor’s office, and they’ll give you back a new organ or some new tissue — a new liver, a new kidney — and you’ll be fixed. And it’s not science fiction. This is very, very real."

Actually, it’s not. Not yet, anyway, and the Atlantic story, written by former biotech researcher Kyla Dunn, makes that clear. Someday, though, it could be. The question is whether government will allow cloning-based medicine to develop in the open — or if, instead, it will merely drive cloning overseas and underground, with consequences we can’t even begin to predict.

SOMEONE — I HAVE long since forgotten who — once wrote that the first step toward creating a police state is building a neighborhood police station. His point wasn’t that we should stop building police stations; quite the opposite. Rather, his point was to show the fallacy of "slippery slope" arguments. We shouldn’t deny ourselves the obvious, practical benefit of a new police station today because of the theoretical possibility that the station could become a garrison in some police-state society tomorrow.

And so it would seem with cloning. There’s something absurd about denying ourselves the possible medical benefits of therapeutic cloning because of the theoretical, if repulsive, possibility that it will be used to manufacture human beings. Surely this is the absurd logical bottom at the end of the slippery slope. The public understands this. According to a CNN/USA Today/Gallup poll taken last November, the public disapproves of reproductive cloning by a margin of 88 percent to nine percent — but by 54 percent to 41 percent, it supports "cloning that is not designed to specifically result in the birth of a human being, but is designed to aid medical research that might find treatments for certain diseases."

At this point, it’s fair to note that I have a personal interest in this. My nine-and-a-half-year-old daughter, Rebecca, has achondroplasia, the most common form of dwarfism. The cause: a genetic mutation that stunts the growth of the long bones in the legs and arms, and that affects the development of other bones as well. Someday it may be possible to create cloned tissue from a baby or fetus with achondroplasia, extract stem cells, fix the genetic defect, and flood the developing child with properly functioning cells that his or her body recognizes as its own. The child might be taller; far more important, he or she might be free of the spinal problems characteristic of achondroplasia, problems that can cause pain, paralysis, even death. (My interest is strictly theoretical, since I assume my daughter is already too old to benefit from such a treatment, which probably lies decades in the future.) I also happen to own stock in two companies whose prospects would improve immediately if the government would give free rein to stem-cell and therapeutic-cloning research.

But these conflicts of interest, if that’s what they are, hardly make me unique. If you have a genetic predisposition to Alzheimer’s disease, if you have a relative whose car accident left her a quadriplegic, if your father and his father died young from heart attacks, then you, too, cannot claim to be disinterested. As Michael Kinsley, who has Parkinson’s disease, recently wrote in a pro-therapeutic-cloning piece for Slate, "It seems more like a bizarre convention than an ethical mandate that a person’s views on a subject should be considered less interesting if his life is at stake."

Of course, all our lives are at stake, even if few of us face the immediacy of that truth the way Kinsley does. Yet in what might be called the highbrow popular press — the opinion pages of our elite newspapers, plus magazines such as the New Republic, the Nation, the Weekly Standard, and National Review, to name some of the most obvious examples — the argument has come down heavily against not just reproductive cloning, but therapeutic cloning as well.

Yes, the Times’ editorial page strongly favors cloning, and it’s not hard to find pro-cloning op-eds. But the heavyweight intellectual artillery has pretty much all been on the opposing side. And it’s come, ideologically, from everywhere: the right, the center, and the left.