O'Connor retired from the Supreme Court last year, replaced by a man. Her departure almost certainly cost Ginsburg's side a victory in an abortion case, decided 5-4 in April, and might have been a factor in a wage discrimination lawsuit the court last week decided, also by a 5-4 vote, against a woman and in favor of her employer.
Ginsburg and O'Connor, the only women among the 110 justices in U.S. history, were not always on the same side. O'Connor, who has her own "I'm Sandra, Not Ruth" shirt, dissented from Ginsburg's very first opinion on the court.
Yet they often were together on issues of particular concern to women, notably abortion rights.
Twice this term, Ginsburg has written sharp dissents that were made more notable because she took the unusual and dramatic step of reading them in court. The last time she read even one dissent from the bench in a term was three years ago.
Calling the court's decision to uphold a nationwide ban on an abortion procedure "alarming," Ginsburg said it "cannot be understood as anything other than an effort to chip away at a right declared again and again by this court — and with increasing comprehension of its centrality to women's lives."
The majority chose not to require doctors to explain different abortion procedures and their risks, she said. "Instead, the court shields women by denying them any choice in the matter. This way of protecting women recalls ancient notions about women's place in society and under the Constitution — ideas that have long since been discredited," Ginsburg said.
Last week, the court threw out a discrimination suit by Lilly Ledbetter, a longtime Goodyear supervisor who was paid thousands of dollars a year less than her male peers. "In our view, this court does not comprehend, or is indifferent to, the insidious way in which women can be victims of pay discrimination," Ginsburg said.
The 74-year-old justice has said more than once that she is lonely without O'Connor on the court, even though the retired justice maintains an office next door to Ginsburg's. In January, asked to assess how O'Connor's retirement would affect the court, Ginsburg told USA Today, "This term may be very revealing."
Ginsburg declined to be interviewed for this story.
Her legal writings have mainly marked Ginsburg as a justice who avoids personal attacks on her colleagues and seeks common ground, said Laura Krugman Ray, a Widener University law professor who has written about Ginsburg's opinions.
"Her voice has been one that has always tried to draw people together," Ray said.
The more personal tone, particularly in the abortion dissent, suggests Ginsburg may not be as comfortable working with Chief Justice John Roberts and Justice Samuel Alito as she was with the late Chief Justice William Rehnquist and O'Connor, Ray said.
The court is going through a transition, in which conservative justices more often have the upper hand and the two new justices have joined seven others who have been together since 1994.
"You wonder with the turnover whether she feels a kind of movement backward and whether that may have energized her," Ray said.
Ginsburg is a former women's rights lawyer who argued discrimination cases before the Supreme Court in the 1970s.
"Both cases touched a raw nerve beyond disagreeing with the majority decision. Both cases reflected a paternalistic attitude toward women in the workplace and their role in society more generally," said American Civil Liberties Union legal director Steven Shapiro. "That is an issue that has been at the core of her professional life for as long as she's been a lawyer."
Leonard Leo, who helped marshal support for the Roberts and Alito nominations, said Ginsburg's opinions in the two cases would be helpful in rallying conservative support if President Bush gets another chance to pick a justice.
"The more vituperative and incendiary it becomes, the more ammunition it provides to the conservative base," said Leo, the executive vice presidential of the Federalist Society, a conservative legal group. Leo said he was offering his own opinions, not speaking for the society.
Ginsburg once said that having two women justices showed that the days of "one-at-a-time curiosities" were over for women at the Supreme Court.
That was six-and-a-half years ago.
More recently she said of O'Connor, "I didn't realize how much I would miss her until she was gone."