Under detailed public questioning for the first time since President Bush tapped him for a seat on the Supreme Court, John G. Roberts Jr. appeared to sound a less conservative note on abortion-related issues than he had in the memos and briefs he wrote as a lawyer in the Reagan and George H.W. Bush administrations.

He offered a surprisingly emphatic endorsement of a constitutional right to privacy -- the basis of the Roe v. Wade decision recognizing a right to abortion, which he seemed to disparage as a young lawyer in the Reagan administration.

And, as if repeating a carefully formulated phrase, he referred time and again to a 1992 Supreme Court ruling upholding Roe as "a precedent entitled to respect."

Yet, ever the careful lawyer, Roberts committed himself to nothing more tangible than a promise to think hard before voting either to uphold or overturn the abortion precedents about which so much controversy revolves. Each time a senator pressed him for a clear statement, up or down, on abortion rights, Roberts refused, saying that to do so would be to declare his views on an issue that the court may face in the future.

The overall impression was of a talented nominee who used his skills to avoid twin perils: revealing nothing of his views or revealing too much.

Tacitly acknowledging that he would have far more latitude to make changes in the law as chief justice than he has now as an appeals court judge, Roberts offered a list of factors he would weigh in deciding whether to overturn a precedent of the court.

Stare decisis is the legal term for the principle that courts should let existing decisions stand for the sake of stability in the law -- and discussion of stare decisis took up much of the hearing, basically as a proxy for the larger issue of abortion.

Roberts told Sen. Arlen Specter (R-Pa.), the Senate Judiciary Committee chairman, that established precedents, including both Roe and the court's 1992 affirmance of Roe in Planned Parenthood v. Casey, should be overturned only when they have proved "unworkable," or when they have been "eroded" by intervening decisions of the court.

As examples of cases in which he thought the court was right to overturn precedent, Roberts cited the court's New Deal-era abandonment of its past cases invalidating economic regulations, and Brown v. Board of Education, the school desegregation case that overturned the court's 1896 validation of racial separation in public facilities.

But those reversals by the court, much debated in their day, are no longer controversial. Roberts's criteria, "while superficially specific, are in fact highly elastic and could be used to overturn Roe and Casey, or to uphold them, depending upon his proclivities," said Michael J. Glennon, a professor of constitutional and international law at Tufts University's Fletcher School of Law and Diplomacy.

No sooner had Specter, a strong supporter of Roe, gaveled the proceedings to order at 9:30 a.m. than he began boring in on Roberts about the abortion precedents, asking whether he agreed with the Supreme Court's view, expressed in Casey, that so many women had come to rely on the abortion right that Roe had to be upheld.

Roberts did not answer directly, but did concede that Casey is a valid precedent "entitled to respect" and that "it is a jolt to the legal system when you overrule a precedent."

Yet he repeatedly brushed aside invitations to say whether overturning Roe -- after being upheld in Casey -- would be too big a jolt.

Specter invoked a change of heart by Roberts's mentor, the late Chief Justice William H. Rehnquist, for whom Roberts clerked in the court's 1980-1981 term. He noted that Rehnquist, a life-long critic of the 1966 Miranda decision that recognized a suspect's "right to remain silent" when questioned by police, ultimately voted to uphold it in 2000.

Rehnquist had concluded that Miranda was "embedded" in the "national culture," and Specter asked whether the same could be said for "a woman's right to choose."

Roberts demurred.

"I think that gets to the application of the principles in a particular case," he said. Like previous nominees to the court, Roberts said, he would refuse to say how he would rule.

In the coming term, the high court will hear a challenge to a New Hampshire law that requires teenagers to notify their parents before they can get an abortion. The question for the court is whether Roe and Casey require the state to permit an exception to the law in cases in which a girl's health may be in danger.

Sen. Herb Kohl (D-Wis.) seemed pleasantly surprised when Roberts told him that he agrees with a 1965 Supreme Court decision declaring "that there is a fundamental right to privacy as it relates to contraception."

Kohl said he was "delighted" to learn that Roberts had just endorsed an opinion which, Kohl said, leads logically to "a woman's right to choose to terminate her pregnancy," the right recognized in Roe v. Wade.

But, once again, Roberts slipped the punch. "That's an area that I do not feel it appropriate for me to comment on," he said.

John G. Roberts Jr. said key precedents should be overturned only when they have proved "unworkable" or been "eroded" by later decisions.