WASHINGTON — Sandra Day O'Connor arrived on an ideologically divided high court during a period of unprecedented challenge to established law on issues such as abortion, affirmative action and criminal justice.

She put her stamp on each of these fields, not by adopting an agenda but by avoiding one, deploying her strategic role to moderate the extremes.

She helped qualify the right to abortion but blocked the efforts of conservative colleagues to overturn it.

She rejected challenges to the use of affirmative action in higher education, instead endorsing its use in narrow circumstances in the interest of "effective participation by members of all racial and ethnic groups in the civic life of our nation."

In what may be her most memorable opinion, she was willing to grant considerable deference to the Bush administration's anti-terrorist detention policies but drew a firm line at the policy of detaining individuals without independent review.

"It is during our most challenging and uncertain moments that our Nation's commitment to due process is most severely tested," she wrote last year for the court in the Iraq-war era case of Hamdi vs. Rumsfeld. "And it is in those times that we must preserve our commitment at home to the principles for which we fight abroad."

Her opinions were by no means her main claim to distinction.

As Thurgood Marshall was the first black jurist on the court, so she was the first woman.

For women lawyers, she symbolized a long struggle first for admission to the profession of law and then for access to the upper reaches. She regaled audiences with her own story: how she graduated third in her class at Stanford Law School but could get in the door of only one major law firm, where she was then duly informed it did not hire women lawyers and offered a secretarial position.

A Western childhood

Sandra Day was born Aug. 26, 1930, in El Paso, Texas. She was raised by her parents Harry A. and Ada Mae Day on a vast ranch, the Lazy B, along the Arizona-New Mexico border.

By her teens, she had learned to dig a well, fix a pickup, fire a rifle, tame a wild horse and dance the Texas two-step. She was a "cowgirl," a "simple cowgirl," as she said later, aspiring for most of her youth to be a cattle rancher.

In 1946 she enrolled at Stanford University in Palo Alto, Calif., at the age of 16. She studied economics, with the idea that the subject would help her manage the ranch. She graduated magna cum laude in 1950 but then enrolled in Stanford Law School at the age of 20, finishing in two years rather than in the usual three.

"I had a professor I really loved, who was himself a lawyer. And I thought one reason he was so effective was his legal background. And because of him, really, I applied to law school. I didn't know where it might lead or if I'd like it," she recalled.

A member of the board of editors of the Stanford Law Review, Day graduated third in a class of 102, two places behind her future Supreme Court colleague William Rehnquist.

At the law school, she met John J. O'Connor III, whom she would marry. They now have three grown sons.

The O'Connors later settled in Phoenix, where he worked for a prestigious law firm while she opened up a small general practice with another woman lawyer.

In 1965, she became an assistant attorney general in Arizona. In 1969, when state Sen. Isabel Burgess left for an appointment to the National Transportation Safety Board, Arizona Gov. Jack Williams plucked O'Connor from the state attorney general's office and appointed her to Burgess' seat, to which she was elected in 1970 and ultimately chosen by fellow Republicans as their majority leader in the Senate.

She disliked legislative politics and at the end of her second full term resigned from the Senate for a judgeship in Maricopa County Superior Court. Then, in 1979, she was offered a seat on the Arizona Court of Appeals.

President Reagan announced her appointment to the Supreme Court on July 7, 1981.

At that time, the court tilted toward the expansive federal power and progressive legislation that had flowered in the past three decades.

Counterbalance sought

O'Connor was Reagan's first appointment, fulfilling a promise he'd made during his campaign to put the first woman on the court, and she was expected to provide a conservative counterbalance.

She did that most notably by joining William Rehnquist and other conservatives in championing a more delicate balance between the reach of the federal government and the rights of the states.

By the early 1990s, with a Republican majority on the court, O'Connor became a crucial voice in landmark decisions that formed the contours of the Rehnquist court's legacy. Federalism, or the idea that many questions should be left to states, is now a fundamental part of that legacy.

O'Connor also helped the court pare back the reach of some federal gun laws and congressional legislation aimed at helping battered women. She has been instrumental in Rehnquist-led efforts to limit federal review of search-and-seizure requirements during traffic stops and the ways in which death-row defendants can challenge their sentences.

But as tough as O'Connor can be on federalism questions, she also has shown a penchant for compassion on many social issues — and a tendency to soften some positions over time. Her social pragmatism has put her at odds with other justices and conservative politicians who favor a more ideological approach.

Honoring precedents

She voted to uphold laws requiring parental notification for minors seeking to terminate pregnancy, but wielded her vote and her voice to stop the court from overturning the core holding of the landmark 1973 ruling in Roe vs. Wade legalizing abortion.

"Liberty finds no refuge in a jurisprudence of doubt," she wrote for the majority in Planned Parenthood vs. Casey in 1992. "Yet, 19 years after our holding that the Constitution protects a woman's right to terminate her pregnancy in its early stages, Roe v. Wade (1973), that definition of liberty is still questioned. We are led to conclude this: the essential holding of Roe vs. Wade should be retained and once again reaffirmed."

In the field of criminal law, she was willing to ease, but not eliminate, the Miranda rule barring use of evidence obtained from a suspect not told of the right to remain silent and the right to counsel.

Writing for a majority in 1985, she allowed for an exception to the evidentiary rule when police fail to warn suspects but act in good faith. "A simple failure (by police) to administer the warnings, unaccompanied by actual coercion," does not hopelessly taint evidence, she said.

But she declared that the court "in no way retreats from the bright-line rule of Miranda."

Similarly, O'Connor has refined her position on affirmative action.

In a 1989 case, she called a Richmond, Va., effort to boost minority contractors unconstitutional because it didn't meticulously document the past discrimination it was trying to remedy. O'Connor was central in developing the court practice of closely scrutinizing any race preferences, and she has tended to frown on them.

But in the key 2003 case challenging the constitutionality of race preferences in admissions at the University of Michigan Law School, O'Connor cast the deciding vote to retain the practice.

"Attaining a diverse student body is at the heart of the Law School's proper institutional mission, and its 'good faith' is 'presumed' absent a showing to the contrary," O'Connor wrote in Grutter vs. Bollinger.

What "distinguishes her approach to judging," wrote conservative scholar and former Justice Department lawyer John Yoo, is "her effort to decide each case on its unique facts and context, her affinity for balancing the costs and benefits of a policy and her desire to leave issues open to be decided another day."

"Positioned exactly in the middle of a polarized nine-member Supreme Court," Yoo wrote, "O'Connor has provided the deciding fifth vote on some of the most controversial issues of our day. ... If someone else had been appointed to her position on the court," he wrote, "our nation might now be living under different rules for abortion, affirmative action, race, religion in school and civil rights. We might well have a different president."

Information on O'Connor's early years on the high court was reported by Knight Ridder Newspapers.