From the moment the medical paper arrived anonymously at the offices of the National Right to Life Committee three years ago, antiabortion activists knew they had been handed a powerful weapon.

The eight-page, double-spaced document described in precise, straightforward language an abortion procedure sometimes used during the second half of pregnancy, at 20 weeks and beyond. A copy of a medical paper that had been delivered at a recent seminar, it was written by an Ohio doctor who had performed the procedure hundreds of times.

It provided what abortion foes had long believed was crucial in turning public opinion their way: a graphic description of one type of abortion they felt would offend many, perhaps most, Americans. In this procedure, the doctor delivered the body of the fetus -- feet first and sometimes still alive -- into the birth canal before collapsing the skull so that the head could be drawn through the opening of the uterus. The medical world called the procedure "intact dilation and evacuation," but antiabortion activists soon coined a new name for it: "partial-birth" abortion.

The activists believed that publicizing the details of the procedure would fuel a national debate, pull many abortion rights liberals to their side and prompt Congress for the first time to ban a specific abortion procedure.

They were right.

President Clinton vetoed the legislation last April. But Congress is gearing up to vote on it again before adjourning at the end of next week. Although proponents of the ban believe they may have the necessary two-thirds vote in the House to override the veto, they acknowledge they still are at least a dozen short in the Senate.

Ongoing efforts to enact the ban have been aided by the considerable weight of leading Catholic clerics, who visited members of Congress last week to lobby for an override, and whose followers have deluged Capitol Hill with millions of postcards.

The issue also has played a role in the presidential campaign. Robert J. Dole, the Republican nominee who supports a constitutional amendment banning nearly all abortions, has said that Clinton's veto "pushed the limits of decency too far." Ten days ago, he told an audience of Catholics, "whether you're pro-life or pro-choice, there is one thing everyone can agree on: Partial-birth abortion is wrong."

Whatever the bill's ultimate fate, the clash over late-term abortions will be remembered as a benchmark in the decades-old abortion debate.

It has forced members of Congress and the general public to confront what happens during abortion -- and most people find such details grisly, no matter what surgical method is used. It also has ignited a discussion of the ethical justifications for abortions performed when a pregnancy is more than half over. Such procedures -- of which the procedure banned by the legislation is only one of several -- make up only 1.3 percent of the 1.3 million abortions done in the United States each year, but they provoke ambivalence and discomfort even among abortion rights supporters.

"This legislation has so mobilized pro-lifers, that the effect of it . . . will strengthen them for a very long time," said Helen Alvare, spokeswoman for the National Conference of Catholic Bishops. "For years, the best we've been able to do in Congress is preserve some funding restrictions. To get from that into the question of abortion itself was a huge leap."

Those on the other side of the debate view the bill's success in Congress as an ominous precedent, and suggest that, if it were law, abortion opponents would try to expand or broadly interpret the ban to cover other kinds of abortions.

"This is the first time Congress has ever attempted to regulate the practice of medicine and abortion," said Kathryn Kolbert, vice president of the Center for Reproductive Law and Policy in New York, an abortion rights group.

Said Lewis Koplik, a New Mexico physician who performs late-stage abortions using a different method: "They don't want less than 1 percent of abortions stopped. . . . They want all abortions stopped." Estimates and Anecdotes

There are no reliable statistics on how many abortions are done each year using the technique that would be banned. Nor is there much information about the women who undergo the procedure or about the condition of the fetuses they carry. As a result, both sides of the debate have selectively used estimates and anecdotes to support their positions.

The National Abortion Federation, an organization of abortion providers, believes 400 to 600 cases of "intact D&E," as the procedure is often called, may be done each year. The National Right to Life Committee, which supports the ban, believes it may be several thousand.

Similarly, there are no reliable estimates on how many American doctors use the technique. Interviews with abortion providers suggest that they are fewer than 20, and perhaps fewer than 10.

Opponents of the ban, including President Clinton, have used patients and data drawn chiefly from the practice of one abortion doctor to portray the procedure as an extremely rare one, used almost exclusively in cases where a woman discovers that her pregnancy threatens her own life or that the fetus is severely deformed. They also have implied that in some cases, it is the only abortion technique that can safely be used.

Interviews with physicians, as well as information gleaned from published documents and congressional testimony, paint a different picture of these late-term abortions.

It is possible -- and maybe even likely -- that the majority of these abortions are performed on normal fetuses, not on fetuses suffering genetic or developmental abnormalities. Furthermore, in most cases where the procedure is used, the physical health of the woman whose pregnancy is being terminated is not in jeopardy. In virtually all cases, there are alternative ways to perform the abortion safely, though perhaps not as safely as when intact D&E is used.

Instead, the "typical" patients tend to be young, low-income women, often poorly educated or naive, whose reasons for waiting so long to end their pregnancies are rarely medical. Only in the small subgroup of women whose abortions are done extremely late -- in the last one-third of gestation -- are most of the fetuses malformed, and most of the pregnancies initially desired.

But if abortion rights advocates have painted a misleading picture of intact D&E, so have proponents of banning the procedure.

Much of their campaign has led people to believe that normal, viable fetuses are regularly being aborted very late in pregnancy -- in the eighth or ninth month -- using this technique. "Virtually every pro-choice American and every pro-life American agrees that aborting a child in the eighth or ninth month the way a partial-birth abortion does is wrong," House Speaker Newt Gingrich (R-Ga.) said in supporting a veto override on "Meet the Press" Sunday.

Most fetuses aborted by the "intact D&E" method are less than 24 weeks gestation; the number done later, when the chances of viability are greater, is very small.

There is no clear-cut moment in pregnancy when a fetus becomes "viable," or capable of surviving outside the womb. Of infants born at 24 weeks gestation, about one-third survive; at 23 weeks, fewer than one-quarter. Most abortion providers will not perform abortions of any type on a normal fetus, carried by a healthy woman, beyond the 24th week of pregnancy; many practitioners set the boundary even earlier.

Antiabortion groups also have cited the fact that the fetus, in some cases, is still alive when part of its body is outside the womb during the procedure. "The difference between the partial-birth abortion procedure and homicide is a mere three inches," Rep. Charles T. Canady (R-Fla.) said last year. Proponents also have argued that fetuses may suffer pain during the procedure.

The usual alternative to intact D&E is "dismemberment D&E," in which the fetal limbs are pulled off the body in utero, sometimes while the fetus is still alive. Proponents of the "partial-birth" abortion ban have not made clear why intact D&E should be outlawed, while "dismemberment D&E" -- used to abort a fetus of similar age while still inside the uterus -- is not. And, if the fetus has sensation -- which is far from certain -- then arguably dismemberment D&E is the more painful procedure.

What's indisputable is that public discussion of this method of ending pregnancy has thrown a spotlight on the anguish and ambivalence that lurks below many -- if not all -- abortions. It has forced doctors, patients and the public to face the "livingness" of the fetus in a way that abortion techniques used early in pregnancy do not. Visual Imagery

Abortion opponents have always relied on visual imagery. They have carried posters depicting the tiny feet of aborted fetuses, and jars with the fetuses themselves. A 1986 antiabortion film, "The Silent Scream," showed an ultrasound image of the supposed agony of a 12-week fetus being aborted. But it was not until they provided drawings of the intact D&E procedure that were descriptive enough to make the point, but not so graphic they couldn't appear in the mass media, that they reached a wider audience.

Within weeks of Martin Haskell's description of the intact D&E procedure at a 1992 National Abortion Federation seminar in Dallas, his paper had been sent to the National Right to Life Committee, said its legislative director, Douglas Johnson. The committee took Haskell's paper, along with some rough sketches of the procedure that had appeared in an antiabortion publication, to an artist who produced more sophisticated drawings. These were circulated within the antiabortion community.

"I was horrified that such a procedure existed," said Canady, who was sent a copy of the paper and later introduced the ban. "It occurred to me that this was something the American people would overwhelmingly oppose if they were aware of it."

In 1993, Haskell said in interviews in two medical publications that he had discovered the procedure by accident, and had performed it more than 700 times. In most cases, he said, the abortions were not done because of a birth defect or a severe maternal illness.

Haskell is no longer granting interviews, "given the harassment he's under," said his lawyer, Kolbert.

The issue landed on Capitol Hill as Congress was debating the 1993 Freedom of Choice Act, a bill that would have prohibited many state restrictions on abortion. Canady argued that the bill would prevent states from banning even late-term abortion techniques, like the procedure described by Haskell, and offered an amendment banning the intact D&E method. But abortion rights supporters had long outnumbered abortion foes in Congress, and Canady's amendment failed by a narrow margin. A procedural fight kept the bill from ever coming up for a vote.

With Republican victories in the 1994 elections, however, more than 40 new antiabortion legislators arrived on Capitol Hill, and the abortion balance changed. And proponents of the "partial-birth" abortion ban believed their chances for a major antiabortion victory were further enhanced by the distastefulness of the late-term procedure.

Antiabortion leaders correctly suspected the issue could split the abortion rights opposition. Sen. Daniel Patrick Moynihan (D-N.Y.), who traditionally had voted for abortion rights, called the procedure "as close to infanticide as anything I have come upon in our judiciary." Previously dependable abortion rights supporters like House Minority Leader Richard A. Gephardt (D-Mo.) and Rep. Susan Molinari (R-N.Y.), similarly decided to support the ban.

It passed 286 to 129 in the House, and 54 to 44 in the Senate.

The emotion that marked the congressional debate has accompanied the issue into the presidential campaign. Dole has pledged that, as president, he would sign the ban on "partial-birth" abortion. He has attacked Clinton's veto, charging it represented his lack of "moral vision."

Clinton has countercharged that his decision was based on defending the health of women whose babies were seriously deformed. "I fail to see why {Dole's} moral position is superior to the one that I took," he said.

Polls suggest that, while Americans generally support a woman's right to an abortion, there is also considerable support for the ban on the "partial-birth" procedure.

Respondents to a Gallup Poll last July were asked if they would favor "a law which would make it illegal to perform a specific abortion procedure conducted in the last six months of pregnancy known as a partial-birth abortion,' except in cases necessary to save the life of the mother." Seventy-one percent said yes.

Supporters of the ban argue that public opinion is shifting as they continue to place advertisements describing the procedure in newspapers and on television. Among the ads is one from a new group of 300 physicians, including former surgeon general C. Everett Koop, which argues that the procedure is never medically necessary.

The quest for public support has shaped strategies on both sides. Abortion opponents focus on the fetus and on the medical details of the procedure. Abortion rights supporters emphasize the rights and health of women and portray the proposed ban as an unwarranted government invasion of privacy. Shaping Strategies

A contentious subtext in this war of images has been the question of why women seek late-term abortions.

"The anti-choice community has done a very good job at painting a picture of a woman who has an abortion as frivolous, irresponsible, one who engages in sex without responsibility," said Kate Michelman, president of the National Abortion and Reproductive Rights Action League.

She and others cited an advertisement run by the National Conference of Catholic Bishops listing examples of reasons a woman could use to obtain a "partial-birth" abortion if the legislation made an exception to preserve the health of the mother. The list included such examples as "won't fit into prom dress," "hates being fat," and "can't afford a baby and a new car."

But the women who have spoken out publicly about their experiences with the procedure have told a different story.

"We are not women popping up in the eighth month saying, I don't think I'll be a mom,' " Claudia Ades told a congressional hearing last November. Ades said she learned from a sonogram when she was 26 weeks pregnant that her fetus had a severely malformed brain and numerous other serious defects.

"These were desperately wanted children, where something went terribly wrong," she said.

In a recent interview, Ades said she and her husband, Richard, who live in Los Angeles, "begged for . . . someone that could fix my baby's brain or the hole in his heart," but were told their child had no chance of survival. She opted for abortion, she said, because she believed her fetus was in pain.

Four different doctors told her intact D&E was the safest way, Ades said. "We knew other options existed," including a Caesarean section, "but they were not considered as safe, as healthy or as appropriate for us. . . . What bothers me is that we have to defend what we did. We believe it was such a humane thing."

Johnson, of the National Right to Life Committee, and others argue that even in the case of severe developmental defects like the Ades fetus, the baby should be allowed to be born. "The premise that in some cases it is necessary to kill the baby to complete a delivery . . . there are no such cases," he said.

Clinton said he would have signed the legislation if it had included an exception for women who faced serious health risks without the procedure. But foes of such an exception argued that it "would gut the bill," in Johnson's words.

While the immediate future of the abortion debate clearly hangs on the November elections, it seems likely that this will not be the last time Congress focuses on a specific procedure.

Rep. Christopher H. Smith (R-N.J.), a leading abortion opponent in the House, said after the House approved the ban late last year that antiabortion lawmakers "would begin to focus on the methods and declare them to be illegal."

For abortion rights supporters, that is a daunting prospect.

"There is no abortion procedure when described that is aesthetically comforting, whether at six weeks or 32 weeks," said Frances Kissling, president of Catholics for a Free Choice. "This is exactly the kind of abortion issue that people don't want to think about. . . . They want women to be able to have this option in such extreme and terrible circumstances, but they know it's not pretty. It has to happen, but it shouldn't be in the newspaper." CAPTION: BENCHMARKS IN THE DEBATE

1973: The Supreme Court ruled in Roe v. Wade and Doe v. Bolton that the U.S. Constitution protects a woman's right to abortion before the fetus is viable. The court also said a state can prohibit abortions after viability unless the mother's life or health is at risk.

1976: Congress adopted the Hyde amendment, prohibiting the use of federal funds for abortions.

1976: In Planned Parenthood v. Danforth, the Supreme Court struck down Missouri provisions that required a married woman to obtain her husband's consent to an abortion, required a physician to preserve the life and health of the fetus and prohibited use of saline amniocentesis as a method of abortion.

1989: In Webster v. Reproductive Health Services, the Supreme Court upheld the major provisions of a Missouri law that prohibited the use of public facilities or public employees to perform abortions and required physicians to test the viability of the fetus before performing an abortion in pregnancies of 20 weeks or more.

1992: The Supreme Court, ruling in Planned Parenthood v. Casey, reaffirmed on a 5 to 4 vote the basic right to abortion established in Roe v. Wade. At the same time, it upheld several Pennsylvania provisions restricting abortion, including those requiring physicians to give patients antiabortion information, a 24-hour waiting period and parental consent for minors. It struck down a requirement that a woman must notify her husband before obtaining an abortion.

1993: Congress debated the Freedom of Choice Act, meant to restore abortion rights restricted by the Supreme Court rulings in 1989 and 1992. Antiabortion legislators argued that the law would prevent states from restricting the late-term abortion procedure later called "partial birth" abortion. The legislation was never brought to a final vote.

1994: Congress approved and President Clinton signed the "Freedom of Access to Clinic Entrances Act," making it a crime to block access to clinics that perform abortions, damage their property or injure or intimidate patients and staff.

1996: President Clinton vetoed the ban passed by Congress on "partial-birth" abortions, saying it failed to provide an exception in cases where the woman faces a severe health risk. CAPTION: Richard and Claudia Ades were told their fetus had no chance for survival. She opted for an intact D&E procedure. CAPTION: Douglas Johnson, legislative director for the National Right to Life Committee, stands near a poster depicting a "partial-birth" abortion.