For a quarter of a century, Sandra Day O'Connor (search) held down the center on the Supreme Court, pleasing liberals by standing firmly for abortion rights but voting with the majority to put Republican President Bush in office.

The first woman to serve on the court, O'Connor has been a crucial vote in holding the middle ground on landmark rulings from abortion to abuses in money and politics.

O'Connor held the center while the court became more conservative in the 24 years since President Reagan appointed her. Still, she often sided with the more conservative justices, as in the ruling that handed Bush victory in the 2000 election (search).

In 1992, O'Connor voted to uphold the 1973 decision legalizing abortion, calling it "a rule of law and a component of liberty we cannot renounce."

She added that some state restrictions on abortion were permissible as long as they did not represent an undue burden on a woman's right to terminate a pregnancy.

She was in the majority when the high court outlawed capital punishment for the mentally retarded. She was in the minority with the conservative wing of the court when more liberal justices ruled that juries, not judges, must make the crucial decisions that can lead to a death sentence.

In the 1980s, the Reagan administration moved to dismantle preferential treatment for minorities. O'Connor was a critical vote in thwarting the administration's plans.

She was the crucial vote when the court upheld affirmative action policies on the nation's college campuses. She played a crucial tie-breaking role as the author of the court's final word on race-conscious legislative redistricting.

"We proceed case by case as they come to us, and not with any overarching objective that the court itself" has developed, O'Connor has said. "We aren't here trying to develop something in the sense of where the country should go."

She voted to uphold a public Christmas display including a creche, but voted to bar a public Christmas display of a creche alone. Her view was that the Constitution prohibits any government action that is intended to send a message endorsing religion. Her vote determined the outcome in both cases.

The only member of the court who had held elective office, she co-authored the majority opinion supporting a law to clean up the system for financing political campaigns. O'Connor was a state senator and county judge in Arizona.

Amid many changes on the court over the years, O'Connor and Justice John Paul Stevens (search) played steadfast roles in the middle.

Early in her tenure, O'Connor expressed hostility to the 1973 ruling legalizing abortion, saying that its central premise — permitting greater state control as a woman's pregnancy proceeds — has "no justification in the law or logic."

But on the much more conservative court of 1992, O'Connor declared, "Our obligation is to define the liberty of all. We reaffirm the constitutionally protected liberty of women to obtain an abortion."

O'Connor later voted with the 5-4 majority in striking down Nebraska's late-term abortion prohibition.

Some Republicans have recently seen potential vacancies, particularly O'Connor's possible retirement, as an opportunity to increase pressure on Bush to nominate a strongly anti-abortion candidate for the next Supreme Court vacancy.