President Bush announced this morning that he will nominate John G. Roberts as the 17th chief justice of the United States.

If confirmed, Roberts will replace William H. Rehnquist, who died Saturday from cancer and will be buried Wednesday. Roberts clerked for Rehnquist in 1980 when Rehnquist was an associate justice.

Bush had selected Roberts in July to fill the seat of retiring Justice Sandra Day O'Connor. Senate committee hearings on the original nomination had been scheduled for Tuesday, but Senate Majority Leader Bill Frist (R-Tenn.) said in a statement today that the hearings have been postponed.

Frist said Sen. Arlen Specter (R-Pa.) will issue a statement tomorrow morning with the new hearing schedule. "We still plan to complete Senate floor action on the Roberts nomination by the start of the new Supreme Court session, October 3, 2005," Frist said in his statement.

Bush's announcement today, which was surprising primarily in its speed, avoids a prolonged public vetting of an entirely new nominee for the chief justice's job. While some liberal organizations have stated their opposition to Roberts' appointment as an associate justice, there was little doubt that he was going to be confirmed for that position, barring any startling revelations.

O'Connor made her resignation effective with the confirmation of Roberts as associate justice. White House spokesman Dana Perino told reporters later in the day that the president called O'Connor this morning and the justice reaffirmed her intention to continue serving until a successor is confirmed. That would allow the court to get started on its new term with a full contingent of nine members.

White House spokesman Scott McClellan acknowledged that there would be a "short delay" in the schedule for Roberts's confirmation hearings.

"Our goal remains to have Judge Roberts in place by the time the court comes back into session. So we're still working from that backstop," McClellan said. He said there had been "some initial discussions" with leaders of the Senate about a new timetable for Roberts' confirmation proceedings.

"Leaders of the Senate have been talking about a short delay of a week or something for the hearings to begin," he said. "Our focus is that he gets in place by the time the court comes into session and we'll work with the Senate to make that happen."

Frist said he thought Roberts would be on the court when it convenes next month. "The President has made an excellent choice; Mr. Roberts is one of the most well qualified candidates to come before the Senate," Frist said in a statement posted on his Web site. "He will be an excellent chief. I still expect Judge Roberts to be confirmed before the Supreme Court starts its new term on Oct. 3."

But Democrats suggested that the change announced by Bush today could complicate the Senate's timetable. "Now that the president has said he will nominate Judge Roberts as chief justice, the stakes are higher and the Senate's advice and consent responsibility is even more important," Democratic leader Harry Reid (Nev.) said in a statement, the Associated Press reported.

As for possible candidates now for O'Connor's seat on the bench, potential candidates Bush scrutinized on the first go-around were his close friend Attorney General Alberto R. Gonzales; his former deputy attorney general, Larry D. Thompson; and a handful of federal appeals judges, including Edith Hollan Jones, Edith Brown Clement, J. Harvie Wilkinson III, J. Michael Luttig, Emilio M. Garza and Priscilla R. Owen.

"For the past two months members of the United States Senate and the American people have learned about the career and character of Judge Roberts," Bush said. "They like what they see. . . .

"The Senate is well along in the process of finding Judge Roberts qualified," Bush said. 'They know his record and his fidelity to the law. . . . I am confident that the Senate can complete hearings and confirm him as chief justice within a month." Bush made the announcement in an 8 a.m. EDT Labor Day television appearance from the Oval Office.

"This had been something in the back of the president's mind in case" if the scenario "came into being," McClellan said. Roberts and Bush met at the White Houst last night and then again at the Oval Office about 7:15 this morning, when Bush offered him the job.

Roberts, 50, is a judge on the U.S. Court of Appeals for the District of Columbia. If he is confirmed before the court session, he would be the youngest chief justice since John Marshall, who was 45 when he took the oath of office in 1801.

Roberts was born in Buffalo, N.Y., but grew up in Indiana. After attending Harvard College and Harvard Law School, Roberts served in the Reagan administration as an aide to Attorney General William French Smith and later to White House Counsel Fred Fielding. He was the principal deputy solicitor general in the administration of George H.W. Bush.

After leaving public office, he went into private practice with the Washington law firm Hogan & Hartson until President Bush named him to the court of appeals in 2003.

He is married and has two children.

Sitting justices have been elevated to the top job on several occasions but it is was the first time an unconfirmed nominee, who formerly clerked for the outgoing chief justice no less, was named chief justice.

"I am honored and humbled by the confidence the president has shown in me," Roberts said, standing alongside Bush in the Oval Office.

"He's a man of integrity and fairness and throughout his life he's inspired the respect and loyalty of others," Bush said. "John Roberts built a record of excellence and achievement and reputation for goodwill and decency toward others in his extraordinary career."