WASHINGTON – President Bush, keeping to his word, moved swiftly yesterday to replace the chief justice of the United States, nominating Judge John Roberts Jr. for the post. It was two days after Chief Justice Rehnquist’s death and one day before Judge Roberts was scheduled to appear before the Senate Judiciary Committee as the president’s choice to replace a retiring associate justice, Sandra Day O’Connor.

The president, who said over the weekend that he would move quickly on a nominee, announced the nomination of Judge Roberts at an early-morning press conference at the White House. Judge Roberts, 50, currently sits on the U.S. Court of Appeals for the Washington, D.C., circuit, but his first job out of law school was to clerk for Justice Rehnquist. If confirmed, he will be the first chief justice to have clerked for his predecessor.

By nominating Judge Roberts for the job, Mr. Bush turned to someone who is said to have won the respect of the court’s current justices and whose record has withstood the scrutiny of liberal interest groups and others who have mined it, seeking political traction. Faced with that difficulty, Democrats have resorted to calling for more documents from Mr. Roberts’s long-ago government service, in an apparent effort either to unearth something controversial or simply to stall for time.

The latter strategy was in evidence yesterday, with Democrats arguing that because the position of chief justice carries more responsibility than the position of associate justice, the hearing should be delayed.

Another contention involved the nominating process itself. Democrats said they are entitled to a full week before beginning a hearing on a new nominee. Judiciary Committee staff negotiated throughout the day, tentatively agreeing that the hearing will start no sooner than Thursday and no later than Monday.

The Senate’s majority leader, Senator Frist, said the decision to delay was made “out of respect” for Justice Rehnquist, whose body will lie in repose at the Great Hall of the Supreme Court building this morning and whose funeral will take place tomorrow. Mr. Frist, a Tennessee Republican, said in a press conference at the Capitol late yesterday afternoon that a final decision on the start of the Roberts hearing would be made this morning, though officials with the Judiciary Committee indicated the hearing would probably start Monday.

Going into the Labor Day weekend, some had suggested a delay in the hearing because of the catastrophic effects of Hurricane Katrina. Senator Schumer, an outspoken member of the Judiciary Committee’s Democratic minority, said Thursday that he thought the relief effort and the hearing on Judge Roberts could proceed in tandem. Yesterday, Mr. Frist said the delay was not related to the hurricane, then amended his statement a few minutes later to say it was.

“It’s Katrina and Rehnquist,” Mr. Frist said. “I think with the death of the chief justice and the lying in repose tomorrow and the funeral, it would be insensitive for us to be initiating the hearing, out of respect for him.”

Faced with a hearing on Judge Roberts as chief rather than associate justice, Mr. Schumer and other leading Democrats said a delay was in order. Employing a phrase echoed in statements issued by other Democrats throughout the day, the senior senator from New York said at a gas station press conference in Manhattan that Mr. Bush had “raised the stakes” by nominating Judge Roberts as chief justice and reiterated his oft-cited criterion for a nominee’s confirmation – that he or she be “in the mainstream.”

Mr. Schumer called on the White House to release additional documents related to Judge Roberts’s time working in the office of the solicitor general. And, like other Democrats, he called on Mr. Bush to announce a nominee to replace Justice O’Connor before the hearing on Judge Roberts proceeds. Mr. Schumer said he has met privately with Judge Roberts three times but has not made a decision on whether to vote for his confirmation.

“I have not made up my mind on Judge Roberts,” Mr. Schumer said. “Truly. I wake up every morning – it’s a very important decision. As a member of the Judiciary Committee, I say, well, ‘What’s in your gut?'”

Mr. Schumer suggested over the weekend that Mr. Bush might ask Justice O’Connor to rescind her plans to retire upon the confirmation of a replacement, and instead to finish out the year on the bench. A Washington attorney who is advising the White House on the hearing of Judge Roberts, Leonard Leo, scoffed at the suggestion.

“I think it’s despicable that Schumer suggested Sandra Day O’Connor serve out another year,” Mr. Leo, who is on leave as executive vice president of the Federalist Society, said. “She wants to spend time with her ailing husband, and she should be allowed to do so.

“The man needs to get a grip on reality,” he said of Mr. Schumer. “I think it’s highly inappropriate to even make the suggestion.”

According to people familiar with White House strategy, Mr. Bush had initially considered Judge Roberts to replace Rehnquist when it was thought that because of his battle with thyroid cancer and his age, 80, he would be the first justice to step down. When Justice O’Connor decided to retire first, Judge Roberts was asked to fill that vacancy instead. Mr. Leo and others said Judge Roberts has the proper temperament to be chief justice, a role that involves handling administrative duties, assigning cases to individual justices, and setting the tone among the justices.

A professor at the University of Notre Dame Law School and another former Rehnquist clerk, Richard Garnett, said Judge Roberts has a temperament similar to that of his former boss. Mr. Garnett said he expects Democrats will push back against the nominee but ultimately vote for confirmation.

“My view before the end of the court’s term last spring and my view today has always been that John Roberts is the best candidate in the country to be chief justice of the United States,” Mr. Garnett said. “On the one hand, he’s a brilliant guy, a sharp lawyer who knows the precedents inside and out. He knows the institution. I’m also confident he has the respect already of the current justices. One reason Chief Justice Rehnquist was so successful was that he had the same combination – someone who is simultaneously very efficient and disciplined but also well-mannered and gentlemanly.”

With the nine-member Supreme Court now facing two vacancies, the president is expected to act quickly to name a replacement for Justice O’Connor. People familiar with the process said they expect Mr. Bush to choose one of three female judges on the Fifth Circuit of the U.S. Court of Appeals whose names first circulated in July: Edith Clement, Edith Jones, or Priscilla Owen. Another potential female nominee is a state court judge in California, Carolyn Kuhl, who withdrew her name as a nominee to the Ninth Circuit of the Court of Appeals after a filibuster by Democrats.

“I would think the pick would be made pretty quickly, but on his own time,” Mr. Garnett said. “I wouldn’t think the president would think he has to give in to some of these demands – that he would have to name someone before they confirm Judge Roberts, for instance. As to who it is, I don’t know. But he ran on picking conservative judges, and I suspect he’ll pick a conservative judge.”