On Capitol Hill, the Senate braced for the possibility of a bitter, divisive and distracting confirmation debate that could sidetrack other legislative business for months.

O'Connor's resignation creates the first Supreme Court vacancy in 11 years, and the confirmation process will play out in the relatively new world of 24-hour news, Internet interest groups, ideologically driven bloggers and polarized politics.

O'Connor, 75, said she was stepping down to spend more time with her husband, John, who has Alzheimer's disease.

"It has been a great privilege, indeed, to have served as a member of the court for 24 terms," she said in a short note that was delivered to President Bush yesterday. "I will leave it with enormous respect for the integrity of the court and its role under our constitutional structure."

Bush praised the departing justice and promised to name a replacement in time for the court's next term, which begins in October. White House aides said he wouldn't announce his choice until after July 8, when he returns from a trip to Europe.

O'Connor specified that her resignation wouldn't take effect until her replacement is confirmed.

The list of possible candidates includes Attorney General Alberto Gonzales, who would be the first Hispanic on the court. The most likely female nominees appear to be appellate court Judges Edith Jones and Edith Brown Clement, both on the 5th U.S. Circuit Court of Appeals in New Orleans.

Other appellate judges who could be tapped for promotion are J. Harvie Wilkinson of the 4th Circuit in Richmond, Va.; J. Michael Luttig, also of the 4th Circuit; Michael McConnell of the 10th Circuit in Denver; Samuel Alito of the 3rd Circuit in Philadelphia; Emilio Garza of the 5th Circuit; and John Roberts of the U.S. Court of Appeals for the District of Columbia.

"The nation deserves, and I will select, a Supreme Court justice that Americans can be proud of," Bush said in a brief statement. "The nation also deserves a dignified process of confirmation in the United States Senate, characterized by fair treatment, a fair hearing and a fair vote."

The president said he would consult the Senate before making his choice, but he offered no clues about his intentions with regard to the court's philosophical balance.

Fellow justices paid tribute to her in statements issued by the court.

Chief Justice William Rehnquist called her a "valued colleague," and added, "I shall miss her greatly." Justices Antonin Scalia and Clarence Thomas, two stalwart conservatives who often disagreed with O'Connor, praised her temperament.

Justice Ruth Bader Ginsburg, the court's only other woman, recalled O'Connor's surprise appearance in a 1996 theatrical production of Shakespeare's "Henry V." O'Connor's character uttered the line "Hap'ly, a woman's voice may do some good."

"Sandra Day O'Connor's voice has done enormous good in the pursuit of justice for all in our land and the world," Ginsburg said.

Activists at both ends of the political spectrum swung into action within minutes of her announcement. O'Connor's planned departure gave groups that had been on alert for a possible resignation by Rehnquist a new, even more pressing reason to rally supporters.

"Her replacement will turn the direction of this court," said the Rev. Rob Schenck, the president of Faith and Action, a conservative Christian group. "We are already praying and working for a nominee that will not waffle as she did."

The liberal group MoveOn began airing a new television ad urging Bush to avoid an "extremist" nominee.

"The president should honor O'Connor and appoint a moderate Supreme Court justice," said Ben Brandzel, the organization's advocacy director. "If President Bush nominates an extremist, it will be up to senators to say no, and the American people will make sure that they do just that."

Sen. Arlen Specter, R-Pa., the chairman of the Senate Judiciary Committee, said: "This is a very polarized country when it comes to the issues which will come to the Supreme Court. I would expect people to want their choice. And everybody can't have their own way."

Specter said he expected to hold confirmation hearings in September but didn't rule out action in August if Bush submitted a name to the Senate.

A resignation announcement from Rehnquist would roil the waters even more, but that doesn't seem as likely as it did earlier this year. The 80-year-old chief justice has thyroid cancer but seems to have rallied recently and looks much less feeble than he did when he delivered the oath of office to Bush in January.

Democrats said they would consider a filibuster, a parliamentary tactic that involves unlimited debate, to block any nominee they considered extremist. Although Senate confirmation requires a bare majority — 51 votes in the 100-member Senate — it takes 60 votes to end a filibuster.

In a deal negotiated with Republicans in May, centrist Democrats had said they would use the filibuster against judicial nominees only under "extraordinary circumstances."

"That's an option," Sen. Christopher Dodd, D-Conn., said when he was asked yesterday about filibustering. "I hope we can avoid that. I don't think any of us relish the idea."

O'Connor's decision to retire capped a remarkable career that mirrored the changing role of women in American life. After graduating third in her class from Stanford University law school in 1950 — two notches behind her classmate Rehnquist — the only job offer she received was a chance to become a legal secretary.

Three decades later, President Ronald Reagan named her to the nation's highest court. The appointment, approved in a unanimous Senate vote, transformed one of the last all-male bastions in government and made O'Connor a reluctant feminist icon.

"I think that I bring to the court differences in background that are more germane than my gender," she told Ladies' Home Journal in 1982. "I think the important fact about my appointment is not that I will decide cases as a woman, but that I am a woman who will get to decide cases."

O'Connor was diagnosed with breast cancer in 1988, but her swift return to the bench after treatment helped quiet resignation rumors then.

Admirers praised her flexibility. Critics accused her of substituting personal opinion and political considerations for the law.

O'Connor has taken issue with suggestions that her ideological independence made her the court's power broker.