Harry Cabluck, Associated Press

U.S. Supreme Court Justice Sandra Day O'Connor is shown before administering the oath of office to members of the Texas Supreme Court, Jan. 6, 2003, in Austin, Texas. O'Connor, nominated by President Ronald Reagan as the first woman on the Supreme Court, announced her retirement yesterday. MORE COVERAGE Analysis: Last day reflected O'Connor's legacy Click here to an interactive package about the members of the U.S. Supreme Court. Click here to a photo gallery tracing Justice O'Connor's legal and judicial career.

WASHINGTON -- Supreme Court Justice Sandra Day O'Connor, 75, the first woman to sit on the nation's highest court, surprised President Bush, her colleagues and millions of Americans yesterday by issuing a three-sentence letter saying she will retire from the bench as soon as her successor is confirmed.

A longtime Republican legislator, activist and judge in Arizona, she became one of the most well-known women in America during her 24-year tenure on the court. She had a reputation in legal circles as a centrist and a swing vote, often breaking the tie for the 5-4 rulings that have marked the court in recent years. She was regarded not as a legal theoretician, but rather as a pragmatist, searching for practical, workable solutions in nearly every aspect of human life.

Bush did not learn of O'Connor's resignation until 9 a.m. yesterday. After what he called a "warm conversation" with her on the telephone, the president went before TV cameras in the White House Rose Garden to praise her as a "discerning and conscientious judge and a public servant of complete integrity."

When Chief Justice William Rehnquist, 80, developed throat cancer last autumn, causing him to miss 11 oral arguments and leaving him increasingly frail, many thought that he would be the first to depart from the current court, which has not had a new member in 11 years.

With liberals and conservatives braced for a bruising battle over the person Bush chooses to replace O'Connor, the president promised to select someone who would make America proud and who would meet "a high standard of legal ability, judgment and integrity, and who will faithfully interpret the Constitution and laws of our country."

Bush has said little about the type of person he might nominate, except that he has mentioned he would like to name the first Hispanic. His former general counsel and good friend, Alberto Gonzales, who is now the attorney general, is Hispanic. But if Gonzales were nominated to the court, he would run into opposition both from liberals, over his advice to the administration regarding the treatment of detainees in the war on terror, and from conservatives, because they think he might be moderate regarding abortion.

There are a number of appellate court judges, such as Emilio Garza Emilio Garza and Edith Jones, both of the 5th U.S. Circuit Court of Appeals in New Orleans, who are now being vetted by the Justice Department as well as Bush's current counsel, Harriet Miers, and Karl Rove, his deputy chief of staff and political guru. The advantage of nominating an appellate court judge is that they already have been confirmed once by the Senate.

O'Connor grew up on her parents' ranch, the Lazy B, a 260-square-mile spread straddling the Arizona-New Mexico border. Because of her lifelong love for Arizona as well as her need to care for her husband, John, who is in the early stages of Alzheimer's disease, some court watchers have long thought that she might be ready to retire.

O'Connor's three sons, Scott, Brian and Jay, and her grandchildren live in Arizona, and she loves playing golf there. She has kept her sprawling home in Paradise Valley, outside Scottsdale, in addition to a Georgian-style brick home in Washington.

Duquesne University Law School professor Ken Gormley, who got to know O'Connor after she gave an address at the school, said what had struck him about her, aside from her intellectual curiosity, was that she was "very much a Western girl" who loved to have fun. Just as she enjoyed taking five years off from her law career to stay home with her children in the late 1950s and early '60s, Gormley said, she is looking forward to spending more time home in Arizona with her husband and family.

Senate Judiciary Committee Chairman Arlen Specter, R-Pa., will be in charge of confirmation hearings for the next justice, even though he himself is fighting cancer. Specter also presided over the contentious hearings for the confirmation of Justice Clarence Thomas, the court's youngest member and a controversial conservative.

Specter yesterday said his committee was prepared to proceed on the nomination at any time.

One of Congress' leading Democrats, Sen. Ted Kennedy, D-Mass., yesterday said he hoped that Bush would consult with the Senate and nominate a moderate, as he described O'Connor. Kennedy warned that, while it is Bush's job to make the nomination, the Senate will take seriously its responsibility to decide whether the president's choice is the right one.

The major issue at the confirmation hearings is certain to be the nominee's stance regarding the legality of abortion, possibly the most politically divisive issue in the nation. Groups both favoring and opposing the procedure yesterday immediately went into hyperdrive, preparing for battle.

Nancy Northup, president of the Center for Reproductive Rights, said pressing for the eventual nominee to declare whether he or she supports the 1973 Roe v. Wade decision, which said the Constitution protects the right to an abortion, will be her most important agenda.

Tony Perkins, head of the Family Research Council, which strongly opposes abortion, said his group often disagreed with O'Connor and would mobilize 20,000 churches to demand that Bush choose a nominee whose views are similar to those of Justices Antonin Scalia and Thomas, the court's most conservative members.

Asked if he would vote to confirm any nominee who does not support Roe v. Wade, Kennedy said he would look at the prospect's support for a variety of civil liberties, but would not make the abortion stance a litmus test. "I will reserve judgment," he said.

Bush strongly opposes abortion but also has said he was not certain that the country was ready to have Roe v. Wade overturned.

Not everyone praised O'Connor yesterday. Robert Bork -- a conservative who, like O'Connor, was nominated to the court by former President Ronald Reagan, but whose confirmation the Senate rejected in 1987 -- said he hoped that Bush would nominate a less-activist judge than O'Connor has been. Interviewed on CNN, Bork said O'Connor "didn't have a firm judicial philosophy" and "lined up with the liberal side" on social issues. There are some judges who depart from a strict construction of the Constitution and others who stick with it, he said. "She departed from the actual Constitution," Bork asserted. "I wouldn't call that moderate; I call it unfortunate."

Throughout her tenure as a justice, O'Connor has been considered a trailblazer, saluted as much for her discipline and hard work as for her opinions. Her name is consistently on lists of the most admired American women. When she had a bout with breast cancer in 1988, she missed no oral arguments. George Mason University law professor Nelson Lund said he had never worked harder in his life than when he was O'Connor's law clerk in 1987-1988, but it was the most rewarding professional experience of his life.

When O'Connor graduated in only two years from Stanford Law School in 1952, placing third in a class that also included Rehnquist, she couldn't initially find a job as a lawyer. She recalled that moment vividly in a recent speech:

"I was shocked. I think I was naive. I had never stopped to think that it might be hard to get a job. [One law firm] partner who interviewed me said, 'Miss Day, how do you type?' And I said, 'Well, fair, not great. I can get by.' And he said, "Well, if you type well enough, I might be able to get you a job as a legal secretary. But we have never hired a woman as a lawyer here. And we don't anticipate doing it."

She found work instead as a deputy county attorney in San Mateo, Calif., until her husband graduated from Stanford a year after she had. He went to Germany as an Army lawyer for three years, and she was a civilian lawyer in the Quartermaster Corps. His service completed, the O'Connors moved to Phoenix, had three sons, and she became a full-time volunteer for many causes, including Republican politics, before founding her own law firm with another lawyer.

When her second son was 5, she became assistant state attorney general and was appointed to fill a vacant seat in the state Senate, eventually becoming the first woman majority leader in the United States.

In 1974, she was elected to the Maricopa County Superior Court. In 1979, then-Arizona Gov. Bruce Babbitt, a Democrat, put her on the state Court of Appeals, and on June 20, 1981, at age 51, she met at the White House with Reagan and found herself nominated to the nation's highest court.

When O'Connor was called to answer questions before the Senate Judiciary Committee, she refused to say how she would decide abortion cases that were certain to confront her on the court. Anti-abortion activists were uneasy about her, but she was confirmed by a vote of 99-0 -- the strongest acclamation any Supreme Court nominee had ever received.

She and her husband, who went to work for a Washington law firm, were immediate hits on the social circuit, sometimes attending several parties a week as she struggled to feel less like an outsider. She founded an early-morning exercise club for women who worked at the Supreme Court, and regularly cooked Saturday brunch for law clerks. She continued to take an annual "adventure" trip with women friends from her college years, hiking, fishing and exploring rough terrain, and she also made numerous trips abroad. She gave dozens of speeches, often to groups of young people, urging more civility within the legal profession.

In the early 1990s, when rumors surfaced that she was thinking of retiring, she batted them down. She decided not to retire during the 2004 presidential election year, partly because she feared that the debate over her successor might be too partisan during an election year.

She has written two books, "The Majesty of the Law," a consideration of current legal issues, and the autobiographical "Lazy B: Growing Up on a Cattle Ranch in the American Southwest," with her brother, Alan Day. In 1999, her granddaughter Courtney, then age 9, collaborated with her to write a book, "Meet My Grandmother ... She's a Supreme Court Justice."

O'Connor has often said she was proud that she hadn't needed to give up "life" to be the first female legislative majority leader in U.S. history and the first woman on the U.S. Supreme Court bench.

When O'Connor was undergoing her Senate confirmation process, she was asked how she would like her epitaph to read. "Ah," she said. "The tombstone question. I hope it says, 'Here lies a good judge.' "