Nearly half the American girls who turn 15 this year will undergo an abortion during their childbearing years, if current rates persist.

And today, three out of 10 pregnancies end in an abortion.

Those are numbers that please no one -- not the Right-to-Life anti-abortionists, not the pro-choice abortion rights advocates, not the millions of other Americans who are troubled or ambivalent about one of the most divisive and volatile political issues of our time.

"I think both sides would agree that fewer abortions is better," said Rachel Gold, a policy analyst for the Alan Guttmacher Institute, a nonprofit group specializing in family planning issues. "If women who needed abortions could get them and the number of abortions declined, I would be thrilled to tears."

Twenty years after Colorado became the first state to legalize abortion and 14 years after the Supreme Court legalized abortion nationally, the controversy shows little sign of letting up.

"We can expect a lot of ferment and contentious debate in the United States on this issue for the foreseeable future," said Douglas Johnson, legislative director and chief lobbyist for the National Right to Life Committee.

Recent developments involving all three branches of the government have helped put abortion is back in the political spotlight: The Reagan administration issued proposed regulations last week that would cut off federal funding for family planning clinics that even mention the word "abortion" as an option for pregnant women who do not want to have a baby. A federal appeals court struck down a Minnesota law requiring women under 18 to notify both parents or obtain a judge's approval before getting an abortion. The law is similar to those in 22 other states, including an Illinois statute that is scheduled for review by the Supreme Court this fall. Legislatures in several other states, including New York, where more abortions are performed than in any other, are considering such laws.Looming in the Senate is the fight over President Reagan's nomination of appellate judge Robert H. Bork to replace Justice Lewis Powell, who retired from the Supreme Court in June. Confirmation hearings on Bork's nomination open before the Senate Judiciary Committee on September 15.

And now, Pope Paul II is about to begin his 10-day, nine-city tour across the U.S. -- a politically-charged visit expected to attract demonstations from supporters of both sides of the abortion issue.The Battle Over Bork

The Bork nomination is a political lightning rod on many issues, including abortion, because of the nominee's outspoken conservative views and because Powell was a swing vote in a deeply divided court that split 5-4 on many recent decisions. These include several decisions upholding the landmark 1973 Roe v. Wade ruling that states may not restrict a woman's right to have an abortion during the first three months of pregnancy.

If confirmed by the Senate, Bork's nomination could reverse the 5-4 Court majority in support of Roe v. Wade, a prospect that delights the pro-life anti-abortionists and appalls pro-choice abortion rights groups. Though predicting the judicial votes of Supreme Court nominees is a tricky business, Bork has left little doubt about his views on Roe v. Wade.

"I am convinced, as I think almost all constitutional scholars are," Bork testified before a Senate Judiciary Committee subcommittee in 1981, "that Roe v. Wade is an unconstitutional decision, a serious and wholly unjustifiable judicial usurpation of state legislative authority."

Reversal of Roe v. Wade, said Stanley K. Henshaw, deputy director of research for the Alan Guttmacher Institute, would lead to "continuing political turmoil," with some states outlawing abortion and others keeping it legal.

"Women with money would if necessary travel out of state, many of them over long distances, to have an abortion," Henshaw said. "Women without money would be forced to bear children they didn't choose to bear."

In 1972, the year before Roe v. Wade, an estimated 172,000 women journeyed to New York from out of state to get a legal abortion.

The nomination is opposed by such groups as the American Civil Liberties Union and the National Abortion Rights Action League and supported by the National Right to Life and the American Conservative Union -- at least in part because of his position on Roe v. Wade.

"We don't know what Bork's views on abortion are per se," said the Right to Life Committee's Johnson. "We support him on the basis of his judicial philosophy and his position on Roe v. Wade."

If the Court overturned Roe v. Wade, Johnson said, most states would enact laws restricting abortion except to save the life of the mother.

"From our point of view it can't be anything but better than the current situation, when the Supreme Court won't permit even the most minimal restrictions on abortion," Johnson said. "Roe v. Wade in practical terms is abortion on demand."

"What the Bork nomination has done," said Kate Michelman, executive director of the National Abortion Rights Action League (NARAL), "is to heighten the understanding that this right {of abortion} is still not a given in this society. It cannot be taken for granted." Limits on Clinics

To supporters of abortion rights, the Reagan administration's proposed family planning rules are seen as an attempt to block access to abortion services and interfere with women's ability to make personal medical decisions. Michelman called the rules "outrageous and very dangerous social policy -- very anti-women and anti-children."

"When I took this job a year and a half ago," Michelman said, "I thought our society was coming to terms with the fact that abortion is a morally acceptable choice. I thought: We're getting there.

"I don't believe that anymore."

The proposed revision of rules for the 17-year-old federal Family Planning Program seeks to accomplish by executive order what Reagan has been unable to persuade Congress to legislate: a drastic restriction of the way some 4,500 federally funded birth-control clinics counsel their clients, most of whom are young and poor.

Under the proposed rules, clinics that offer both federally funded birth control counseling and privately funded abortion counseling would have to use physically separate offices for the two services -- an expensive change for many clinics. Many Planned Parenthood clinics, for example, would be affected.

The proposal is strongly supported by the National Right to Life Committee.

"We don't believe the counseling in those facilities is in any way neutral," Right to Life's Johnson said. "In our view it would be very naive to believe that they're not promoting abortion in those situations where there's a de facto merger between a federally funded family planning clinic and an abortion mill." "

Johnson accused many such clinics of telling women "that abortion is just another option. They tell them it's no big deal, it's easier than having a tooth pulled, and we can provide it right here in this very office for $200."

Nothing infuriates the pro-choice groups more than the anti-abortionists' claim that they take abortion lightly and impose it on unsuspecting women.

"A lot of people out there seem to think that because a million and a half women in this country get abortions every year, it's a decision entered into lightly and easily by these women," said the Guttmacher Institute's Gold. "People don't realize that most women who decide to have an abortion do so after great pain, usually after great thought, not happily, and in consultation with several people.

"It's not like going out and buying a candy bar." Rhetoric Wars

One battlefield in the political war over abortion which has not changed significantly in the last decade is the language itself, as groups stake out their positions as forcefully and advantageously as they can. Thus, anti-abortionists commonly call themselves "pro-life," cloak their movement in terms such as "Right to Life" and "Moral Majority" and refer to abortion as "legalized murder." Supporters of legalized abortion, resenting the implication that they are anti-life, call themselves "pro-choice" and frame the issue in terms of "reproductive freedom," and a woman's "right to privacy."

One side talks about reproductive options and the freedom of choice, the other about legalized murder and the sanctity of life.

Those who oppose abortion tend to focus their attention on the fetus, usually referring to it as an unborn child, and championing its "right to life." Those who support abortion as a legitimate option tend to focus their attention on the pregnant woman, her "right to control over her own body" and her right to terminate a pregnancy that she did not intend and does not want.

"I'd like to see a day when women didn't have to face unplanned pregnancy, but the social conditions don't allow that yet," said NARAL's Michelman. "The debate over abortion in this country is really about the value of women's lives as full, equal, moral beings. It's not just a legal-political question. It's about justice for women and their ability to bring children into the world in a thoughtful, deliberate way."

Both sides can cite political victories in the 20-year struggle over abortion and both claim to have the backing of public opinion. Surveys suggest that while most Americans feel some ambivalence about the issue of abortion, a majority supports at least limited legalization.

National surveys also show that Catholics have about the same abortion rate as other Americans and that many women facing unwanted pregnancies choose an abortion even though they oppose abortion in the abstract, Guttmacher's Henshaw said.

A survey of 1,664 women who underwent an abortion in 1981, found that 10 percent thought abortion was immoral, 17 percent were not sure, and the rest thought it was moral.

In response to another question in the survey, 10 percent of the women who had abortions agreed that abortion is equivalent to murder.

"When a person is facing a pregnancy she doesn't want, that reality can overcome a lot of other values," he said. Abortion Rate Stable

The escalation of the political debate comes at a time when the number of abortions in the U.S. appears to have stabilized.

Nearly 1.6 million American women underwent abortions in 1985, according to the latest figures available from the Alan Guttmacher Institute, which conducts the most up-to-date and complete survey available. Nearly 3 percent of American women of childbearing age had an abortion in 1985. Women ages 18 and 19 had the highest abortion rate.

Both the number of abortions and the abortion rate -- the number of abortions per 1,000 women of childbearing age -- have remained fairly constant since 1980. The Centers for Disease Control reported last month that the number of reported abortions in 1983 declined for the first time since 1969, but the CDC-reported decline was only about 2 percent, or 35,000 abortions. The Guttmacher Institute has found no decline in its surveys.

The two groups gather their abortion statistics in different ways. The Guttmacher Institute's data are based on surveys of all known abortion providers -- including hospitals, clinics and doctor's offices. The CDC figures are based on data from state health agencies, not all of which report abortions. For example, California reports only abortions done in hospitals -- about 10 percent of the estimated total. The California total reported by the CDC for 1983 was nearly 100,000 lower than the Guttmacher Institute estimate.

Just what these statistics mean is difficult to determine. Many factors affect the number and rate of abortions, including the number of women of childbearing age (which is declining as the baby-boomer bulge in the population hits middle age), sex education, the availability of abortion providers (91 percent of rural counties lacked an abortion provider in 1985) and changing patterns of birth control use (such as the declining use of "the pill" in the 1970s and growing use of voluntary sterilization). Underreporting of abortions, particularly in unmarried teenagers, is another statistical problem.

"It's a very, very complicated thing," said Dr. Tedd V. Ellerbrock, an obstetrician-gynecologist and medical epidemiologist in the pregnancy epidemiology branch at CDC. "It requires a whole team of us just to sort out what are the important questions to figure out what these statistics really mean."

What is significant, Ellerbrock said, is the fact that both the abortion rate and the ratio of abortions to live births have been declining since 1980.

Unintended pregnancies continue to occur at a high rate among American women. In 1982, 65 percent of women aged 40 to 44 had had at least one unintended pregnancy in their lifetime. And 23 percent of women in that age group had undergone an abortion, the Alan Guttmacher Institute has calculated.

For younger women, barring a major change such as the reversal of Roe v. Wade, the abortion rate will be even higher.

Some 46 percent -- nearly half -- of the 15-year-old females in the United States can be expected to have at least one abortion by the time they reach age 45, said Guttmacher Institute researcher Henshaw.

The statistic is based on CDC and other data on incidence of abortions by age group, adjusted for underreporting, and takes into account that some women will have more than one abortion. For every 1,000 women reaching childbearing age, at current rates there will be an estimated 760 abortions during their childbearing years, he said. That is about three abortions for every four American women.

"We regard the problem of abortion as just one part of the problem of unintended pregnancy," Henshaw said. Political Action

What both abortion rights advocates and anti-abortionists can agree on is that they're in for a long political struggle on an issue that seems to defy easy resolution.

Both sides have already geared up for an aggressive campaign over the Bork nomination to the Supreme Court. The Senate Judiciary Committee has been flooded with mail, mostly from conservative constituents. But now the anti-Bork mail is beginning to come in. The National Abortion Federation is launching a postcard campaign and asking women who have had abortions to write to the Senate committee.

Meanwhile, the pope's visit has prompted demonstrations and counter-demonstrations, with pickets in front of the Vatican Embassy by Eleanor Smeal, former president of the National Organization for Women and by Nellie Gray of the March for Life.

John Cavanaugh-O'Keefe, founder of the Pro-Life Nonviolent Action Project, has organized the "We will stand up" campaign, asking abortion clinics to close down during the pope's visit. At the same time, the National Abortion Federation is encouraging clinics to stay open.

It's all part of the political roller-coaster that has marked the abortion issue from the beginning.

"Politically, it's been up and down since 1973," said NARAL's Michelman. "Now we're at a high political pitch, with the Bork nomination."

"From where we sit the momentum is in our direction," said the Right to Life Committee's Johnson. "But we know there are tremendous obstacles, because we know a lot of people don't agree with us."