The chief justice's chair that John G. Roberts Jr. occupied yesterday carries with it the titular leadership of the Supreme Court and the federal judiciary as a whole.

But as far as the actual decisions of the court go, Sandra Day O'Connor's seat is perhaps more critical. For years, O'Connor, 75, has been the court's swing vote on the moral and social issues that most inflame both left and right.

And that is why legal analysts are so closely scrutinizing White House counsel Harriet Miers, a former Texas lottery commissioner with little in the way of a paper trail, who was named to replace O'Connor.

Thanks to O'Connor, abortion rights have remained essentially intact, affirmative action in university admissions has survived and official support for religion has been limited to those expressions that can pass a complicated constitutional balancing test.

The 1981 appointee of President Ronald Reagan has moved steadily left in recent years, voting to strike down the death penalty for the mentally retarded and displays of the Ten Commandments on public property.

During the tenure of the late Chief Justice William H. Rehnquist, the nine-member court split between four generally liberal justices, three generally conservative ones and two moderate conservatives, O'Connor and Justice Anthony M. Kennedy, who usually voted with the conservatives but sometimes defected on social issues.

The substitution of one conservative, Roberts for Rehnquist, did not change that basic dynamic.

Replacing O'Connor with a committed conservative would create a 5 to 4 majority for the right on most issues as Kennedy is probably to the right of O'Connor.

Among the issues on which conservatives had hoped to see short-term change was abortion. The basic right to abortion, recognized in the 1973 case of Roe v. Wade, is not yet in play.

But the court will soon have opportunities to chip away at it. It will hear a case later this term on whether states must include a health exception in laws that require pregnant teenagers to notify their parents before seeking an abortion. And the Bush administration has asked the court to uphold a 2003 federal ban on the late-term abortion procedure called "partial birth" by critics.

The late-term abortion case offers the clearest instance of an issue on which O'Connor's vote would make the difference: In 2000, the court voted 5 to 4 to strike down a similar state ban. O'Connor voted with the four liberals in the majority.

Miers has no paper trail in part because she has no judicial experience -- unlike 61 of the 109 justices so far, including O'Connor, a former state judge.

Instead, the bulk of her time was spent in private law practice, prior to her chairing the Texas state lottery commission, and then moving on to Washington when Bush became president in 2001.

The last nominee to come to the court with no judicial experience was Lewis F. Powell, appointed by President Richard M. Nixon in 1972.

A crucial difference between Powell and Miers, however, was that Powell did not know Nixon. He was a national leader in the bar, having been president of the American Bar Association, said David Yalof, a professor of political science at the University of Connecticut who specializes in nomination politics.

By contrast, Miers both has no judicial experience and is a close confidante of the president who nominated her.

Yalof said that the closest analogy to Bush's selection of Miers may be President Lyndon B. Johnson's choice of Abe Fortas in 1965. Fortas had been Johnson's personal lawyer earlier in his career, just as Miers had represented Bush in Texas.

"LBJ was accused of cronyism and the opposition is going to try to make that argument with Harriet Miers," Yalof said, adding that he did not know himself if such charges would be legitimate.

Yet there are some similarities between Miers's resume and that of O'Connor.

O'Connor made a name for herself in Arizona not only as a lawyer but also through nonpartisan civic activities such as the Junior League and the National Conference of Christians and Jews. A Republican party activist, she was nevertheless known as a consensus-seeker whose pet cause as a young government lawyer was the reform of state mental hospitals.

Similarly, Miers, 60, first became known in Dallas not for her involvement in conservative causes, but for leadership in the city and state bar, as well as nonprofits such as the Community Council of Greater Dallas, Child Care Dallas and the YWCA.

In 1992, she received an award from the Dallas chapter of the American Jewish Committee, in recognition for her role in bringing black and Hispanic leaders into the Dallas Bar Association's board of directors, and for helping lay the groundwork for the AIDS Legal Clinic in Dallas.

"Harriet was just a trial lawyer who had done a lot in the community and was highly thought of in the community," said Darrell Strelitz, executive director of the Dallas AJC. There were "tons of Democrats" on the committee that nominated her for the award, Strelitz said.

In July, White House counsel

Harriet Miers escorted Supreme Court nominee John G. Roberts Jr.

to meet with President Bush.

Miers now is the nominee to

replace Justice Sandra Day O'Connor, who held the crucial

fifth vote on many key decisions.