No living person has had more influence over current abortion law in America than retiring Supreme Court Justice Sandra Day O'Connor. So the fight over her successor is sure to draw heat from this intensely polarizing issue.

But can O'Connor's successor have as much impact as she had on abortion, by undoing what O'Connor has done? Even some prominent critics of abortion rights express doubts.

President Bush and Vice President Cheney, for example. Both say they would like to see an end to abortion in the United States, yet they also contend that the change is unlikely to come by Supreme Court fiat. Bush disappointed some antiabortion activists earlier this year when he minimized the importance of "changing laws," compared with "chang[ing] hearts."

That is, in part, O'Connor's legacy. There was a time, in the late 1980s, when she might have overturned the court's prior rulings on the constitutional right to choose an abortion. Instead, she used her key vote to embed the right to abortion far more deeply than it was when she joined the court in 1981.

Another example: Judge Michael McConnell of the U.S. Court of Appeals for the 10th Circuit. He is often touted as a possible Supreme Court justice. Before becoming a judge in 2002, McConnell was one of America's leading antiabortion thinkers and once described the original abortion rights decision, Roe v. Wade, as an "embarrassment."

McConnell testified at his confirmation hearing, however, that the right to an abortion early in pregnancy is now "as settled as any issue can be in constitutional law."

Judge Emilio M. Garza of the U.S. Court of Appeals for the 5th Circuit is another name often mentioned for the high court. He has also written blunt denunciations of the 1973 Roe v. Wade decision. Yet Garza has applied O'Connor's post-Roe framework to deciding abortion cases that come before his court.

Those cases in turn set precedents for other decisions, and with each new case O'Connor's framework for abortion rights cases has been steadily buttressed. Today, the legal search engine FindLaw identifies 150 different circuit court of appeals decisions from across the country that rely in part on O'Connor's most important abortion decision, Planned Parenthood v. Casey, authored with Justices Anthony M. Kennedy and David H. Souter.

That case effectively replaced the complicated mechanics of Roe v. Wade with a more streamlined set of rules. As O'Connor saw it, the law recognizes a woman's right to an abortion early in pregnancy as a matter of her liberty to decide her own private matters. States can regulate abortion, she ruled, but not to an extent that creates an "undue" burden on the woman.

Later in pregnancy, when the fetus is "viable" outside the womb, more stringent restrictions are permissible -- unless they endanger the woman's health.

Even if O'Connor is replaced with a strong antiabortion justice, the court will still have a slim five-vote majority in favor of the basic abortion rights in Planned Parenthood v. Casey. What might change is the extent of the state limits the court would tolerate. O'Connor's vote was the deciding factor, for instance, in a 2000 case that struck down Nebraska's ban on the procedure called "partial birth" abortion by critics.

Republican attorney Mit Spears, a veteran of the Reagan-era judicial nomination wars, said that he expects to hear a lot about abortion in the fight over O'Connor's successor -- but not because it is the most important issue on the table.

"We're beyond that debate as a legal matter," he said. "Now the issue is just a bloody shirt."

That it is. Few issues are as galvanizing for the extreme wings of the political spectrum as abortion, according to Andrew Kohut, director of the Pew Research Center for the People and the Press. Only about one-third of all Americans would like to see Roe v. Wade completely overturned, Kohut's polls indicate, but that number jumps to two-thirds of conservative Republicans. Among liberal Democrats, it falls to less than one in six.

Some of the best-funded advocacy groups, left and right, place abortion high on their agendas and central in their direct-mail appeals. James Dobson, leader of the conservative Focus on the Family, has called abortion "the most significant moral issue of our time" and described the United States as "the bloodiest nation in history."

On the other hand, such liberal groups as People for the American Way and the Alliance for Justice have cast all Republican nominations to the Supreme Court in the past generation as potential death blows to reproductive rights.

Why is abortion such a potent issue, even as the law has solidified and opinion polls steadily show that a strong majority of the country supports at least a limited right to terminate an early pregnancy?

One reason is what Dobson said: For abortion opponents, this is a profound moral issue. To them, a fetus is a person, so an abortion is a murder. They compare Roe v. Wade to the infamous 1857 Supreme Court decision, Dred Scott v. Sanford, which held that "the enslaved African race . . . had no rights which the white man was bound to respect; and that the negro might justly and lawfully be reduced to slavery for his benefit. He was bought and sold and treated as an ordinary article of merchandise."

Last year, during the presidential campaign debates, Bush referred to Dred Scott as proof that the Supreme Court's mistakes must be undone, and many commentators believed he was signaling to the antiabortion movement that he would set his sights on Roe in his second term.

But Dred Scott was overturned by the Civil War and the 13th Amendment. For abortion foes, a clearer example of the Supreme Court reversing itself is the 1896 decision upholding racial segregation, Plessy v. Ferguson. Like the right to abortion, segregation was woven into subsequent decisions, only to be overturned in 1954 by Brown v. Board of Education. But that step was taken by a unanimous court -- not by the vote of a single new justice.

Another reason abortion can so polarize Supreme Court nominations is that other "culture war" or "values" issues are closely related to it. The history of abortion law is part of the broader history known as the sexual revolution.

In the early 1960s, married couples challenged a Connecticut law that banned the sale of contraceptives. This led the Supreme Court in 1965 to define a "right to privacy," though the Constitution did not speak of one directly.

This right was then extended, in 1972, to unmarried people seeking birth control. The link between marriage and the right to sexual privacy was cut.

The following year, in Roe, "privacy" became the basis for abortion rights. Finally, in 2003, the right of sexual privacy (redefined with O'Connor's help as a "liberty") was extended to gay men and lesbians.

Thus, the core concept in abortion law -- of sexual liberty or privacy -- pops up across a red-hot range of values-laden issues, from abortion and gay rights, to premarital and extramarital sex, and from there to the sanctity of traditional families and the role of women in society.

Greg Shaw, a political scientist at Illinois Wesleyan University, predicted that the abortion issue will be uppermost in the administration's thinking about O'Connor's replacement. "Because symbolic issues are very important to people," he explained. People "don't have to think much about them, they're very gut-level, very visceral."

And yet, he also expressed doubts that a summer of fire over abortion will produce much substance. Having studied the consistency of American opinions on this subject, "I would imagine Americans are not going to change the way they think about abortion as a result of having to think about it through this lens," Shaw said.

Assistant polling director Claudia Deane contributed to this report.

Abortion rights proponent Luqman Clark of Arlington and antiabortion activist Jessica Meunier of Fitchburg, Mass., lift signs with contrasting messages outside the Supreme Court in January during the annual demonstration that marks the court's controversial 1973 decision in Roe v. Wade.