John G. Roberts Jr., the nominee of President Bush to be the new chief justice of the United States, pledged today to approach every case with an open mind and to promote a modest role for the Supreme Court.

Appearing before the Senate Judiciary Committee on the opening day of his confirmation hearings, Roberts repeatedly compared a judge's role to that of a baseball umpire and implicitly rejected the "judicial activism" that Bush and Republican lawmakers have said federal judges should avoid.

Roberts's five-minute opening statement came after more than three hours of speeches by the Judiciary Committee's 18 senators. In opening the first confirmation hearing for a chief justice nominee in nearly two decades, the senators turned their statements into something of a debate on the role of the Supreme Court and on how deeply to probe the nominee about his views on issues and judicial philosophy.

Speaking without notes after taking an oath before the panel, Roberts said he had been inspired by the late Chief Justice William H. Rehnquist, the jurist he has been nominated to succeed. Rehnquist, who died of cancer Sept. 3 at age 80, formerly employed Roberts as a clerk and was his mentor.

"My personal appreciation that I owe a great debt to others reinforces my view that a certain humility should characterize the judicial role," Roberts said. "Judges and justices are servants of the law, not the other way around. Judges are like umpires. Umpires don't make the rules; they apply them."

Both "make sure everybody plays by the rules," he said. "But it is a limited role. Nobody ever went to a ball game to see the umpire." He added later in his brief speech, "I will remember that it's my job to call balls and strikes, and not to pitch or bat."

Roberts said his appreciation and understanding of the Supreme Court increased after he left the Justice Department, where he argued cases as a young lawyer on behalf of the government before the Supreme Court, and entered private practice, where he argued against the government. All he had to do, faced with the power of the federal government aligned against this client, was to convince the high court that the law was on his side, he said.

"It is what we mean when we say that we are a government of laws and not of men," Roberts said. "It is that rule of law that protects the rights and liberties of all Americans. It is the envy of the world, because without the rule of law, any rights are meaningless."

Roberts said, "I come before the committee with no agenda. I have no platform. Judges are not politicians who can promise to do certain things in exchange for votes."

If confirmed as chief justice, Roberts pledged, "I will confront every case with an open mind."

He concluded, "If I am confirmed, I will be vigilant to protect the independence and integrity of the Supreme Court, and I will work to ensure that it upholds the rule of law and safeguards those liberties that make this land one of endless possibilities for all Americans."

In opening statements, the committee's Republican chairman and its top Democrat said they plan to question Roberts closely about his views on congressional prerogatives, expressing concern about what they view as the Supreme Court's erosion of the legislative branch's authority.

Other Republicans and Democrats on the committee differed sharply on what kind of questions Roberts could properly be asked in the hearings. Republicans said questions about his views on cases and controversial issues should be off limits, while Democrats made clear they would press the nominee to explain his positions in view of the great influence he is likely to wield as a lifetime appointee.

One leading Democrat on the committee, Sen. Joseph Biden of Delaware, said that if he were to base his decision only on Roberts's previous statements and writings, he would have to vote against confirmation. So far, no senator has said publicly that he plans to vote against Roberts when the committee decides whether to send the nomination to the full Senate, probably later this week.

Roberts, 50, who became a federal judge less than three years ago, was originally picked by President Bush to succeed Justice Sandra Day O'Connor, who announced her retirement in July, effective upon the confirmation of her successor. But after the death of Rehnquist, Bush decided to make Roberts his nominee to replace the chief justice and to name a successor to O'Connor later. Roberts was named in 2003 as a judge on the U.S. Court of Appeals for the District of Columbia Circuit.

If, as expected, he wins confirmation, Roberts would become the 17th chief justice and would be the first new member of the court in 11 years.

Before the hearing began, nearly two dozen demonstrators gathered outside the Supreme Court. They carried signs that said, "Confirm Roberts" and "Stop Abortion Now."

Meeting in the historic Caucus Room of Washington's Russell Senate Office Building, the 10 Republicans and eight Democrats on the Judiciary Committee faced the task of questioning a man who has carefully avoided expressing personal views in past confirmation hearings on such hot-button issues as abortion rights and affirmative action.

The first day of the hearings was expected to be devoted mainly to opening statements by the nominee and all 18 committee members.

Republicans encouraged Roberts to stick to a principle of not answering questions on matters that might come before the court, while Democrats wanted to examine as much as possible the views of a man who could shape the Supreme Court's direction for generations.

"Don't take the bait," said Sen. John Cornyn (R-Tex.). "Do not head down that road, but do exactly what every nominee of every Republican president and every Democrat president has done: Decline to answer any question that you feel would compromise your ability to do your job." He said the vast majority of the Senate "will not punish you for doing so."

The chairman of the Judiciary Committee, Sen. Arlen Specter (R-Pa.), said in an opening statement that he would not ask Roberts directly whether he would vote to overturn Roe v. Wade, the Supreme Court's 1973 decision legalizing abortion. But he said senators might approach the issue by trying to question Roberts on his views on precedents.

When asked about the issue during his 2003 confirmation hearing, Roberts said Roe v. Wade was "settled law."

Specter said he planned to question Roberts closely on what he regards as the Supreme Court's tendency in some cases to usurp congressional power.

"I'm concerned about what I bluntly say is the denigration by the court of congressional authority," Specter said. He cited a ruling that struck down a portion of a law to protect women against violence on grounds of lawmakers' faulty reasoning. He complained that the ruling suggested "congressional incompetence."

Specter also said he plans to ask Roberts about a memorandum -- one of more than 70,000 documents released by the White House dealing with Roberts's service as a lawyer in the Reagan administration -- in which Roberts expressed a view that Supreme Court justices should be limited to 15-year terms.

The ranking Democrat on the committee, Sen. Patrick Leahy of Vermont, said in opening remarks, "The judiciary is the most isolated branch of our government from public accountability. So this hearing is the only opportunity for the American people to examine what kind of justice John Roberts will dispense."

Leahy told Roberts that he is the first nominee to the Supreme Court of the 21st century. If confirmed, Leahy said, "you could serve through the administrations of the next seven or eight presidents," deciding matters that affect not only all Americans today, but "also our children and grandchildren."

He said a key issue for senators is "whether the Supreme Court will effectively check the greatly enhanced presidential power that has been amassed in the last few years."

Sen. Dianne Feinstein (D-Calif.) said she will ask Roberts about "the constitutional right to privacy" as it deals with abortion rights.

"I am concerned by a trend on the court to limit this right and thereby to curtail the autonomy we have fought for and achieved," said Feinstein, the only woman on the committee. "It would be very difficult . . . for me to vote to confirm someone whom I knew would overturn Roe v. Wade."

She said she remembers "what it was like when abortion was illegal in America," saying that as a college student in California she knew one woman who went to Tijuana "for a back-alley abortion" and another "who killed herself because she was pregnant."

Sen. Edward M. Kennedy (D-Mass.) said the Senate "was not intended to be a rubber stamp for a president's nominees to the Supreme Court." The nomination comes at a time when "long-established rights to privacy are under heavy siege," Kennedy said.

Sen. Biden said Roberts is "right in the middle" of a genuine intellectual debate about whether the Constitution should be interpreted narrowly or more expansively. He said there are elected officials, such as Rep. Tom DeLay (R-Tex.), the House majority leader, "who have been unsuccessful in implementing their agenda" in Congress and have now "poured resources into trying to change the court's view of the Constitution."

Biden said he believes the Constitution "recognizes a general right to privacy" and empowers the government to "stamp out discrimination wherever it occurs."

He told Roberts, "If I look only at what you have said and written, I would have to vote no" on confirmation. He cited memos in which Roberts referred to a "so-called right to privacy" and "dismissed gender discrimination as merely a 'perceived problem.'"

But Sen. Jon Kyl (R-Ariz.) said that "not every question that a senator might think of is legitimate" for the confirmation hearing.

"One is not qualified by the court by virtue of his position on issues but rather by his ability to judge fairly," Kyl said. "I will defend your refusal to answer any question that you believe is improper. . . ."

Sen. Lindsey Graham (R-S.C.) said the real issue in the Roberts confirmation process is "whether the Senate will allow President Bush to fulfill his campaign promise to appoint a well-qualified strict constructionist to the Supreme Court." Declaring that "elections matter," Graham said Bush had made no secret of his intentions in two presidential campaigns and that he has "lived up to his end of the bargain with the American people."

One of the Senate's more liberal Democrats, Charles Schumer of New York, said he wants to determine whether Roberts is "in the conservative mainstream," which he described as acceptable, or whether he is "an ideologue" who would use the court to impose his personal views.

Americans "need to be confident that your claim of judicial modesty is more than easy rhetoric, that your praise of legal stability is more than lip service," Schumer said. "They need to know above all that, if you take the stewardship of the high court, you will not steer it so far out of the mainstream that it founders in the shallow waters of extremist ideology." He said it was "not enough to say you will be fair."

Sen. Sam Brownback (R-Kan.), a staunch opponent of abortion, said in his opening statement that Roe v. Wade and Doe v. Bolton were two 1973 rulings "based on false statements which invented a constitutional right to abortion."

Since then, "nearly 40 million children have been aborted in America, 40 million lives that could be amongst us but are not, beautiful, innocent faces that could bless our existence and our families and our nation, creating and expanding a culture of life," Brownback said. "If you're confirmed, your court will decide if there is a constitutional right to partially deliver a late-term child and then destroy it." But even if Roe v. Wade is overturned, he said, "it does not ban abortion in America. It merely returns the issue to the states. . . ."

The most emotional speech of the day came from Sen. Tom Coburn (R-Okla.), a physician and freshman senator, whose voice cracked as he pleaded for unity.

"My heart aches for less divisiveness," he said, "less polarization, less finger-pointing, less bitterness, less mindless partisanship. . . ."

Three senators who are not Judiciary Committee members introduced Roberts -- two of them, Richard Lugar (R) and Evan Bayh (D), from his home state of Indiana. Sen. John Warner (R-Va.) said in a third introduction that Roberts has the strongest qualifications of any of the more than 2,000 judicial nominees he has seen in 27 years in the Senate.

Warner said attorneys who have worked with and against Roberts "typically speak with one voice when they tell that you that dignity, humility and a sense of fairness are the hallmarks of this nominee."