If the retirement of Justice Sandra Day O'Connor opened a gap at the ideological center of the Supreme Court, the death of Chief Justice William H. Rehnquist removed the anchor of its right wing.

Yet in what is suddenly a much more complex process of replacing not only O'Connor but also Rehnquist, President Bush has an opportunity to shore up the court's conservative bloc and entrench it.

Rehnquist's replacement will probably serve for many years to come; if the new justice's views remain conservative over that time, it will mean the effective perpetuation of a Rehnquist-like vote on the court long after Bush is gone.

"Even if you appoint someone who is identical to Rehnquist in every respect, you're talking about someone who is about 50 rather than 80," said Richard Lazarus, who directs the Supreme Court Institute at Georgetown University Law Center.

Bush must also be sensitive to the fact that replacing a chief justice is not quite the same as replacing an associate. The titular head of the federal judiciary needs to be temperamentally suited to organizing the strong-minded jurists who sit with him on the Supreme Court and on lower courts.

"The next chief justice, whether chosen from within or outside the court, has a very high mark to follow," said A.E. Dick Howard, a professor of law at the University of Virginia. "Ideology aside, it's going to be difficult to run the court any better than he did," referring to Rehnquist. "We will look back on the Rehnquist court as one of the smoothest in the court's history."

And the president must fill the void quickly to minimize the disruptions to the judicial process that can occur when the Supreme Court is short-handed.

Unlike the departure of O'Connor, who often voted with Rehnquist and other conservatives on the court but who defected on social issues, the death of Rehnquist means the loss of an unequivocally conservative vote -- and voice -- on the court.

Of the justices remaining, only two, Antonin Scalia and Clarence Thomas, can be called sure votes to overturn Roe v. Wade, should a direct challenge to that abortion rights ruling come before the court. Only Scalia, Thomas and Anthony M. Kennedy are firmly opposed to affirmative action.

Almost all of those mentioned as likely successors to Rehnquist would probably vote as he has.

The one exception could be Attorney General Alberto R. Gonzales. His record suggests he could be slightly to Rehnquist's left on issues such as affirmative action, which he has supported in public statements, and Roe, which he cited as valid precedent while a justice of the Texas Supreme Court.

He could be to the right of Rehnquist on issues relating to anti-terrorism efforts; Rehnquist voted against the Bush administration's policy of denying court hearings to prisoners at Guantanamo Bay, Cuba, which Gonzales supported.

An analysis of recent voting patterns at the court for the last five terms (not counting the last 12 cases decided this term) by attorney Kevin Russell of the law firm Goldstein & Howe shows that Rehnquist has cast a deciding vote in only 14 percent of 361 cases decided during that period.

These included such high-profile cases as Bush v. Gore, which ended the 2000 election, and Zelman v. Simmons-Harris, in 2002, which permitted the use of publicly funded school tuition aid for private religious education. But most of them were relatively minor.

Also, Russell's study showed that there is relatively little room to Rehnquist's right on the court. He cast a deciding vote against Scalia and Thomas only five times.

If Rehnquist had been replaced by "an ideological twin" of Scalia during the court's most recent term, Russell noted, only two of his votes would have been different. But in one of those, plugging in a Scalia clone would have favored a criminal defendant's claim, and in one of them it would have favored prosecutors.

Some academic observers thought that Rehnquist's leadership skills and his human touch would be harder to replace than his ideology.

Certainly it was personal qualities that his colleagues on the court emphasized in their public statements yesterday, with Justice Ruth Bader Ginsburg, in a formulation echoed by other justices, praising him as "a plain speaker without airs or affectations." Ginsburg said that Rehnquist "fostered a spirit of collegiality among the nine of us perhaps unparalleled in the Court's history."

At the moment, the court is down to eight members, as O'Connor's retirement takes effect upon the confirmation of a successor.

The last time the court functioned with only eight members for an extended period was in 1987, after Justice Lewis F. Powell retired on June 26 of that year. After the defeat of Robert Bork as a replacement, followed by the withdrawal of a second nominee, Douglas Ginsburg, a successor, Anthony M. Kennedy, was not confirmed until Feb. 3, 1988.

The court had only eight members for 363 days between the resignation of Justice Abe Fortas on May 14, 1969, and the confirmation of his successor, Harry A. Blackmun, on May 12, 1970.

Each time, a short-handed court labored to avoid 4 to 4 tie votes, which automatically affirm a lower court's ruling but create no binding legal precedent.

In an oral history recorded in 1995, Blackmun recalled how he was greeted with a pile of 47 petitions for certiorari that had been held to see whether he would supply a fourth vote needed to hear the cases.

The court now has similar options to prevent tie votes and other anomalies; for example, it can postpone hearing certain close cases until it is back at full strength.

"If [the White House] wants the court not to suffer the way it has in past cases, they've got to get moving," said Dennis J. Hutchinson, a professor of law at the University of Chicago. "On the other hand, if they say, 'We're talking about a 20-, 25-year cycle,' that's more important than the court being stalled in the water for a few months."

By law, Rehnquist's position will be filled on an interim basis by Justice John Paul Stevens, 85, the senior associate justice.

It is the first time an associate justice has filled in for a chief justice who died in office since Justice Hugo Black replaced Harlan Fiske Stone for 63 days in 1946.