She was the deciding vote, again and again, on some of the most critical and contentious issues to reach the Supreme Court in the past two decades, including affirmative action, capital punishment and the disputed 2000 presidential election.
And if the court's next term gets under way in October with her replacement confirmed and on the bench, following the timetable that President Bush set out Friday, the retirement of Justice Sandra Day O'Connor could have almost immediate implications.
Already on the calendar for this fall is a string of politically charged cases which, under the court's current lineup, could well have come down to O'Connor's vote. The cases include the Bush administration's challenge to the nation's only right-to-die law, Oregon's Death with Dignity Act, and an appeal of a federal court order striking down as unconstitutional a New Hampshire law requiring parental notification when teenagers seek abortions.
The court has also agreed to decide in its coming term whether colleges and universities that receive federal funding may bar military recruiters, setting up a clash between government funding and free speech, and raising issues of gay rights.
A coalition of law schools has argued that they have a right to refuse to associate with military recruiters, pointing to the First Amendment and what they view as the Pentagon's discriminatory policies on gays and lesbians. The Pentagon contends that because the schools accept federal funding, they have an obligation to allow the military to recruit on campus.
The case is being closely watched by civil liberties groups, which praised O'Connor's willingness to distance herself from administration policies in such cases as Hamdi v. Rumsfeld, in which she ruled that a U.S. citizen seized as an "enemy combatant" in the fight against terror can challenge his detention in U.S. courts.
Death penalty issues also could hang in the balance. O'Connor, who in 2001 publicly questioned how fairly capital punishment was administered, sided in recent years with a majority that struck down the death penalty for the mentally retarded, but dissented this year from a 5-4 decision banning juvenile executions.
The court agreed last week to hear the appeal of a death row inmate from Tennessee who claims that new DNA evidence could exonerate him in the 1985 rape and murder of a female neighbor whose husband also was a suspect in the crime.
O'Connor wrote a 2003 decision that threw out the death sentence of Maryland inmate Kevin Wiggins, saying trial lawyers in his 1989 case failed to present mitigating evidence about horrific childhood abuses.
Two of the court's other conservatives, Justice Anthony M. Kennedy and Chief Justice William H. Rehnquist, joined the 7-2 decision.
The court's two most conservative members - and the judges that President Bush has said he would most want to replicate in appointing a new justice - wrote a scathing dissent. Saying that the majority's reasoning ranged from "the incredible up to the feeble," the dissent from Justices Antonin Scalia and Clarence Thomas concluded: "Today's decision is extraordinary - even for our `death is different' jurisprudence."
On such divisive questions, it was O'Connor during her 24 years on the nation's high court who was closely watched as the bellwether. Her retirement, court scholars say, could produce a seismic shift on some of the most high-profile constitutional questions to come before the court - regardless of who is nominated to her seat.
"Her departure is a huge loss to anyone who has wanted the court to continue restraining congressional power over the states," David Garrow, a law professor and Supreme Court historian with Emory University, said in offering an example of O'Connor's influence. "If I was the attorney general of Oregon, I would be very disappointed ... because I just lost a vote."
How great the court's shift will be in the coming years depends largely on who is named to replace O'Connor. But her departure is expected to leave a mark regardless, because of her unique case-by-case approach to the job and her efforts to decide cases on the narrowest, most fact-specific grounds.
That approach meant O'Connor never fit squarely with the court's most conservative or its most liberal members, and she moved between the two blocs often as she earned her reputation as the court's most critical swing vote.
"Justice O'Connor fully earned her reputation as a centrist; she was a conscientious jurist and, in a number of key cases, stood up for individual rights and against a radically conservative vision of the constitution," said Steven Shapiro, legal director for the American Civil Liberties Union.