Senators have discussed abortion, civil rights and other issues in the opening days of debate over John G. Roberts Jr.'s nomination to the Supreme Court. But the hottest topic is the question about questions, as senators wrangle over what should be asked and answered -- or not answered -- in confirmation hearings later this summer.

Democrats say Roberts is obligated to describe his views on major political and social issues, including whether the landmark ruling on abortion rights was "correctly decided." Republicans are counseling him to rebuff such queries and to reveal no more of his thinking than necessary.

The debate is pivotal because senators have only one chance to grant or deny a lifetime appointment to the high court, and insight into how a nominee might rule on major issues depends largely on his willingness to volunteer his thoughts and beliefs. A review of decades of Supreme Court confirmation battles, however, finds the dispute unresolved.

Answers offered or withheld by successful nominees in the past have varied widely. And minority-party senators have repeatedly learned that, with few exceptions, there is nothing they can do about a stonewalling nominee but complain, hold news conferences and vote nay, only to see the majority put the nominee on the court.

The ground rules for asking and answering questions "are kind of ad hoc," said Carl Tobias, a University of Richmond law professor who monitors the court. Republicans and Democrats have alternately argued for and against expansive questioning over the years, he said, and "they invoke the history they need for the particular occasion."

The abortion issue illustrates the erratic pattern. John Paul Stevens, the first Supreme Court nominee tapped after the 1973 Roe v. Wade ruling, received not one question about abortion. The next nominee, Sandra Day O'Connor, told senators she had made a mistake 11 years earlier by voting in the Arizona legislature to decriminalize abortion, but she otherwise declined to say how she might rule.

The next three successful nominees -- Antonin Scalia, Anthony M. Kennedy and David H. Souter -- refused to discuss their abortion views despite pressure from committee Democrats. Following them in 1991 was Clarence Thomas, who testified that he had never expressed a view, even in a private conversation, about Roe v. Wade, leaving many Democrats incredulous and angry.

Once on the court, Scalia and Thomas signaled opposition to Roe, while the others backed it. Partly because liberals felt deceived by Thomas, the ground had shifted by the time Ruth Bader Ginsburg, nominated by a Democrat, went before the Judiciary Committee in 1993. She strongly endorsed the right to abortion, calling it "central to a woman's life, to her dignity." The Senate confirmed her 96 to 3.

The most recent justice confirmed, Stephen G. Breyer, in 1994, was a bit dodgier. He told the committee that Roe "has been the law for 21 years" and signaled no wish to change it.

That is essentially the answer Roberts gave in 2003, when he was confirmed to the U.S. Court of Appeals for the District of Columbia Circuit. He called Roe "the settled law of the land," a response that many senators expect to hear at his Supreme Court hearings.

Three liberal Democrats on the committee voted against Roberts in 2003, saying he sidestepped too many questions. One of them, Sen. Richard J. Durbin (Ill.), recently met with Roberts and urged him to be more forthcoming this time.

But Republicans point to telling facts: The Senate unanimously confirmed Scalia and Kennedy despite their non-answers on Roe. And despite the three committee Democrats' pique two years ago, the full Senate confirmed Roberts to the appellate court without opposition, by voice vote.

The lesson, some Republicans say, is that reticence is safer than trying to explain one's views. Senior Judiciary Committee member Charles E. Grassley (R-Iowa) said he told Roberts last week that he had studied nominees who won confirmation easily and "it seems like the less they say to the committee, the better off they are."

Committee Chairman Arlen Specter (R-Pa.) says senators are free to ask any question they like, and the nominee is free to answer fully, partially or not at all. "Nominees," Specter said, "tend to answer just as many questions as they have to in order to be confirmed."

Democrats hope to push Roberts to illuminate his views on civil rights, the environment, disabled people, abortion and other subjects. Judiciary Committee member Charles E. Schumer (D-N.Y.) says the public's desire for meaningful answers from nominees ebbs and flows. A recent Washington Post-ABC News poll found that a solid majority of Americans want Roberts to explain his views on abortion.

"The time in history, and Sandra Day O'Connor's removal, means the focus on real questions will be greater than it has ever been, because she was a swing nominee at a time of a divided court and a divided country," Schumer said.

All senators agree that nominees should not signal how they might rule on a case that could come before them on the court. Beyond that, opinions vary dramatically. Some senators say a nominee may comment on decided cases, and Schumer has told Roberts he will ask him if Roe v. Wade "was correctly decided."

But Sen. John Cornyn (R-Tex.) said in a Senate speech yesterday that insisting on such questions "will undoubtedly force [Roberts] to prejudge a case that is currently pending on the court's docket," a reference to a New Hampshire case on parental notification when minors seek abortions. Senators note that even long-established cases -- such as the one that allowed "separate but equal" public facilities for blacks and whites -- can be overturned by subsequent rulings.

Democrats warn that nominees can imperil themselves by rebuffing too many questions. They point to appellate court nominee Miguel Estrada, who was blocked by a Democratic filibuster on those grounds in 2003. Democrats pushing back include Sen. Edward M. Kennedy (Mass.), the committee's elder statesman. "The only way to know whether nominees have an activist judicial philosophy," he said in a recent speech, "is to find out what their judicial philosophy is. . . . A resume is no substitute for answering questions about whether the nominee respects the basic rights and freedoms on which the nation was founded."

Specter, a GOP moderate, seems to seek a middle ground. "I wouldn't say, 'Are you going to uphold Roe?' " he recently told NBC News. "But I would ask a nominee, 'What is your view of stare decisis?' " -- Latin for "to stand by that which is decided."

"When you have a decision which has been in effect for decades and people have come to rely upon it," Specter said, "what kind of circumstances, how extraordinary must they be" to overturn it? In his 2000 book, "Passion for Truth," Specter wrote: "The Senate should resist if not refuse to confirm Supreme Court nominees who refuse to answer questions on fundamental issues."

While conservatives are leading the effort to limit questions for Roberts, they have been the victims of circumscribed hearings in the past. Souter's noncommittal replies, for example, prompted major abortion rights groups to urge his rejection on the assumption that he would seek to overturn Roe. Instead, Souter became a reliable defender of abortion rights.

In fact, seven of the current nine justices were appointed by Republican presidents, but conservatives note that some -- especially Stevens -- proved to be much more liberal than expected. The seven GOP nominees could have given the court a solid conservative majority, Grassley said in an interview, but "we ended up with two liberals, two moderates and three conservatives."

Research editor Lucy Shackelford contributed to this report.

Sen. Arlen Specter (R-Pa.) says the scope of the judiciary panel's questions is unlimited but the nominee may choose to answer fully, partially or not at all.