Although Rehnquist had been battling thyroid cancer for nearly a year, the White House, members of the Senate and his fellow justices had no warning that his condition had become dire in the past few days, officials said.

Justice David H. Souter said he was "flabbergasted."

It was not clear whether Bush already had in mind a nominee to replace Rehnquist. Nor did the White House give any hint of whether Bush would elevate one of the current associate justices to be chief justice instead of nominating a judge from outside the court — perhaps even Judge Roberts — to the post.

But Bush, who had previously said his goal was for Roberts to be confirmed in time for the beginning of the court's new term on Oct. 3, signaled that he did not intend to wait long to announce a decision.

"There are now two vacancies on the Supreme Court, and it will serve the best interests of the nation to fill those vacancies promptly," he said. "I will choose in a timely manner a highly qualified nominee to succeed Chief Justice Rehnquist."

In an interview later, Bartlett said, "There is a short list, and it's being worked very hard."

Bush could elevate a sitting associate justice, most likely Antonin Scalia or Clarence Thomas, to chief justice, and then nominate a judge from outside the court to fill the resulting vacancy. He could renominate Roberts as chief justice and choose someone else for O'Connor's seat. Or he could fill the chief justice's seat from outside.

Speculation about filling a vacancy from outside centered on the same group of federal appeals court justices that had been on Bush's short list when he chose Roberts to replace O'Connor. That list included the Attorney General Alberto R. Gonzales, Judges Edith Brown Clement and Edith H. Jones, both of the 5th U.S. Circuit Court of Appeals, and Judges J. Michael Luttig and J. Harvie Wilkinson III, both of the 4th U.S. Circuit Court of Appeals.

Clement sits in New Orleans, a fact that could give her selection special resonance as that city reels in the aftermath of Hurricane Katrina. Republicans, including Sen. John Cornyn of Texas, and former White House officials said Bush will be strongly inclined to choose either a woman or a Hispanic nominee like Gonzales.

White House officials began planning for the nomination almost immediately after they received word of Rehnquist's death late Saturday night, Bartlett said. On Sunday, he said, Bush and top aides including Harriet Miers, the White House counsel, Andrew H. Card Jr., the chief of staff, and officials from the Justice Department met to discuss the matter.