Justice Sandra Day O'Connor, a self-described "simple cowgirl" who became the first woman on the Supreme Court, announced her retirement yesterday after nearly a generation as the court's pivotal voice on society's most burning issues, from abortion and race to capital punishment and terrorism.

O'Connor, 75, stunned the capital with her simple three-sentence letter hand-delivered to the White House yesterday morning, touching off what both parties expect to be a media-saturated struggle to choose her successor. The consequences could hardly be greater, given the precarious ideological balance on the court and O'Connor's role for many years as the deciding vote.

Her announcement threw off balance the political and legal establishment in Washington, which had expected Chief Justice William H. Rehnquist to retire when the court's term ended Monday and then stood down after he did not. President Bush first got word Thursday that a sealed envelope from the court would arrive the next morning but did not know from which justice, and he kept even that news secret from everyone in the West Wing but Vice President Cheney and two aides.

Within minutes of the letter's arrival, all manner of activists, lawmakers and administration officials -- many heading off for the holiday weekend -- scrambled to activate long-prepared battle plans. Bush and senators from both parties rushed to the microphones to lay down markers. Interest groups launched television advertising and sent millions of e-mail messages.

The coming nomination will be the first in 11 years and the first of the Internet era, an opportunity for Bush to remake the court in a decisive fashion. Scheduled to fly Tuesday to Europe for an international summit, Bush will not name a nominee until after he returns Friday, the White House said, but advisers have already produced a list of more than half a dozen candidates for him to consider, mainly conservative federal appellate judges.

Appearing in the Rose Garden an hour after receiving her letter, Bush praised O'Connor as "a discerning and conscientious judge" and promised to pick a successor "who will faithfully interpret the Constitution." Bush, whose selections of lower-court nominees provoked a bitter months-long standoff in the Senate, pledged to consult lawmakers and called Senate Democratic leaders yesterday with a promise to meet after his European trip.

"The nation deserves, and I will select, a Supreme Court justice that Americans can be proud of," the president said. "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."

But liberal critics wasted no time going on the attack even without a nominee identified. The advocacy group MoveOn.org released a television ad asking, "Will George Bush choose an extremist who will threaten our rights?"

Sen. Edward M. Kennedy (D-Mass.), whose early opposition to Robert H. Bork helped doom his nomination to the Supreme Court in 1987, threatened to do the same to a Bush candidate. "If the president abuses his power and nominates someone who threatens to roll back the rights and freedoms of the American people," Kennedy said, "then the American people will insist that we oppose that nominee, and we intend to do so."

Conservative allies of the president responded with their own media onslaught, including a webcast mocking Democrats in a satirical news show reporting that they were opposing George Washington and Benjamin Franklin after Bush named them to the court. "Today the battle is joined," said a statement from the Committee for Justice, a group founded to support the White House, singling out 12 Senate Democrats from conservative states "who will be held accountable" if their party blocks Bush's choice.

"This is the most important resignation and nomination . . . in our lifetime and probably more than that," said Jay Sekulow, chief counsel to the American Center for Law and Justice, an advocacy group founded by evangelist Pat Robertson. "It's very, very significant. Justice O'Connor is the pivotal vote on so many key cases. This has got gargantuan" implications, Sekulow said.

The nominee could reshape not only the court but also the president's place in history. "The person will serve for life and long outlive" Bush's presidency, said C. Boyden Gray, who was White House counsel to Bush's father and founded the Committee for Justice. "Presidents are in some cases defined by their nominations."

At the same time, the nomination could overshadow the rest of Bush's domestic agenda for months, particularly his plan for Social Security, which has foundered upon rock-solid Democratic opposition and Republican ambivalence. Some allies suggested it could even provide a way to quietly drop the Social Security effort until after next year's midterm elections.

The nomination battle could be the first of two, if Rehnquist, who is 80 and has thyroid cancer, decides to retire. The earliest Bush is likely to name a nominee to replace O'Connor is July 11, advisers said, and some think he might wait until after the Senate's August recess to avoid leaving a nominee as a target for opponents.

Former attorney general Edwin Meese III, one of four outside advisers who plotted strategy with top White House officials last week, said in an interview yesterday that Bush has three options on timing: name a nominee as quickly as possible and ask senators to remain in August, request that the Senate Judiciary Committee come back from recess early to begin hearings, or wait until the end of the recess to announce a nominee. "That's one thing I know will be discussed very carefully," Meese said.

Bush gave no indication of his plans except to say he will "choose a nominee in a timely manner" in hopes of filling the vacancy by the court's next term, which begins Oct. 3. O'Connor said she will remain on the court until her replacement is confirmed. Senate aides said hearings could begin in late July, with senators recalled in August.

Aides said Bush plans to bring files on candidates to study during his European trip and will interview finalists when he returns. He will also meet with Senate Majority Leader Bill Frist (R-Tenn.), Minority Leader Harry M. Reid (D-Nev.), Judiciary Committee Chairman Arlen Specter (R-Pa.) and Sen. Patrick J. Leahy (Vt.), the committee's ranking Democrat.

Bush and his team have prepared for a Supreme Court vacancy for four years and the field of candidates has long been vetted. The most heavily favored candidates, outside White House advisers said, are Judges J. Michael Luttig of the U.S. Court of Appeals for the 4th Circuit and John G. Roberts of the District of Columbia Circuit. Senior officials said Bush has not ruled out his longtime friend and adviser Alberto R. Gonzales, the current attorney general, who would be the nation's first Hispanic justice but is viewed with suspicion by some conservatives, who see him as too moderate on abortion and affirmative action.

Because Bush will be filling O'Connor's seat rather than Rehnquist's, some outside advisers said he may want to focus on appointing another woman, such as Judges Edith B. Clement and Edith Hollan Jones of the U.S. Court of Appeals for the 5th Circuit. Some conservatives are promoting two appellate judges who were just confirmed in the past few weeks after much controversy, Priscilla R. Owen of the 5th Circuit and Janice Rogers Brown of the D.C. Circuit.

Other appellate judges believed to be possible candidates include Samuel A. Alito Jr. of the 3rd Circuit, Michael W. McConnell of the 10th Circuit, Emilio M. Garza of the 5th Circuit and J. Harvie Wilkinson III of the 4th Circuit.

Almost all of the known candidates are thought to be more conservative than O'Connor, who made her mark in the middle of the court as a pragmatist balancing the two extremes. A former Arizona state legislator, O'Connor was appointed to the high court by President Ronald Reagan in 1981 and cast the deciding vote in cases that banned state-sponsored prayer at high school graduations, preserved narrowly drawn affirmative action in university admissions, overturned a ban on late-term abortions known as "partial birth" by foes, and required the Bush administration to grant hearings to detainees in the terrorism fight.

By virtue of her position on so many 5 to 4 cases, she became perhaps the most influential woman in public office in U.S. history and was regularly named in public surveys as one of the most admired women in the United States. Yet her rulings often infuriated conservatives, who had hoped for more fidelity to Reagan's views.

"She shaped law in a number of very important areas through a very pragmatic approach of deciding the cases as they came to her," said Stuart Delery, a former O'Connor clerk. "The idea that she wasn't applying the law is wrong. She was not making decisions for decisions' sake. But at the same time, whether it was because of her background in politics or not, she had a sense of what the consequences would be" of her rulings.

O'Connor offered no public explanation for her decision to step down after 24 years on the bench, but court spokeswoman Kathy Arberg cited age and family considerations. "She's 75 years old, and she needs to spend time now with her husband," Arberg said. O'Connor has told friends that her husband, John Jay O'Connor III, has Alzheimer's disease. Once off the bench, she will have more time to care for him and more opportunities to make money on the lecture circuit and corporate boards.

O'Connor participated in a regular Friday exercise class at the court's gym between 8 and 9 a.m. yesterday and gave not a hint to the bombshell that was about to drop.

White House officials said they did not know O'Connor's plans until yesterday morning. Pamela Talkin, the marshal of the Supreme Court, called White House counsel Harriet Miers about noon Thursday to say she would be delivering a sealed envelope the next day but did not say who the envelope was from or what its contents were. Miers, according to a White House account, called Chief of Staff Andrew H. Card Jr. -- on vacation in Maine -- to tell him, then interrupted Bush's lunch with Vice President Cheney to inform them.

No one else in the White House, including Deputy Chief of Staff Karl Rove, was told until yesterday, officials said. Talkin told Miers by telephone shortly before 9 a.m. that the letter related to O'Connor. Miers informed Bush, who met with Cheney, Rove and counselor Dan Bartlett. The letter arrived at 10:15 a.m.

"It has been a great privilege, indeed, to have served as a member of the Court for 24 Terms," O'Connor wrote. "I will leave it with enormous respect for the integrity of the Court and its role under our Constitutional structure."

Bush then called O'Connor at 10:18 a.m., and they had what aides described as an emotional conversation. "For an old ranching girl, you turned out pretty good," Bush told O'Connor, according to spokesman Scott McClellan. As they talked, Bush also told her, "I wish I was there to hug you."

But O'Connor declined to come to the White House, telling Bush she was heading to the airport for a flight out of town, according to another senior administration official, and she disappeared from public view.

News commentators gather outside the Supreme Court to speculate on who might replace Justice Sandra Day O'Connor, appointed in 1981.