Justice Sandra Day O'Connor, the first woman to serve on the Supreme Court, is retiring after nearly 24 years as an associate justice.
In a letter to President Bush, O'Connor, 75, said her resignation would take effect upon the confirmation of her successor.
"It has been a great privilege, indeed, to have served as a member of the court for 24 terms," she wrote. "I will leave it with enormous respect for the integrity of the court and its role under our constitutional structure."
In another statement, O'Connor said she looks forward to spending more time with her husband, who reportedly has Alzheimer's disease.
O'Connor's retirement could start to shift the court's vote on key issues including abortion, affirmative action and the death penalty for the mentally retarded, analysts said, because O'Connor has cast key votes in recent cases on those and other issues.
Bush said this morning that he would have a nominee ready in time for Senate confirmation before the next Supreme Court term begins in the fall.
"The nation deserves, and I will select, a Supreme Court justice that Americans can be proud of," Bush said. "The nation also deserves a dignified process of confirmation in the United States Senate, characterized by fair treatment, a fair hearing and a fair vote."
He added: "America is proud of Justice O'Connor's distinguished service, and I'm proud to know her."
Considered a conservative at the time of her nomination by President Reagan in July 1981, O'Connor's subtle shift to the center enhanced her influence on the court and solidified her position as one of the most powerful women of her time.
A Supreme Court justice since Sept. 25, 1981, O'Connor's influence has been felt on issues ranging from abortion and affirmative action, to sexual harassment and terrorism.
"She had a very aggressive view of the role of the courts as instruments of social justice and national policy," said Jeffrey Rosen of the George Washington University Law School. "She saw the court in general, and herself in particular, as the appropriate body to decide all these contested questions of national policy."
After her nomination, O'Connor was confirmed unanimously by the Senate.
For much of her tenure, the court has been divided between conservatives and liberals. Chief Justice William Rehnquist, Antonin Scalia and Clarence Thomas often were seen as the most conservative justices. John Paul Stevens, David Souter, Ruth Bader Ginsberg and Stephen Breyer were seen as more liberal.
O'Connor and Anthony Kennedy were seen as moderates, and O'Connor often provided the pivotal fifth vote in the most hotly contested cases.
On affirmative action, O'Connor sided with the liberals, saying that race can be a factor in university admissions.
On terrorism, she wrote the 5-4 opinion in the first case about 9/11 to reach the high court. She said that even in a time of war, all Americans have a right to see a lawyer, whether or not they are suspected of siding with our enemies.
Angered Both Sides
Diane Sawyer once asked O'Connor on ABC News' "Good Morning America" about her role as the court's "swing vote."
"It seems to me that often people writing for public consumption try to find some hook to hang a slogan on," O'Connor said, "and that's not a slogan that I think is very apt."
Apt or not, many court-watchers refer to the high court as the "O'Connor court," and her record in 5-4 decisions speaks for itself -- she has angered both sides in some of the most emotionally charged cases. Liberals were incensed when she allowed restrictions to be placed on the right to abortion. Years later, she infuriated conservatives by upholding a controversial late-term abortion procedure.
Classmate of Rehnquist
O'Connor was born March 26, 1930, in El Paso, Texas, as Sandra Day. She married John Jay O'Connor III in 1952. The couple had three sons -- Scott, Brian and Jay.
O'Connor received her bachelor's degree and law degree from Stanford University, graduating third in her 1952 law school class -- the same class in which Chief Justice William Rehnquist finished first.
She worked as a prosecutor or private attorney in California, Arizona and Frankfurt, Germany, and as an assistant attorney general of Arizona. She later served in the Arizona state Senate before becoming a judge in 1975, ultimately serving on the Arizona Court of Appeals before her nomination to the U.S. Supreme Court.
Part of her legacy on the Supreme Court will be her vote to stop the Florida recount that cleared the way for George W. Bush to win the 2000 presidential election.
O'Connor is philosophical about her role in the center of many firestorms.
"We wish we could go through life not deciding cases that caused anguish among a wide percentage of our citizens," she said. "But that isn't always going to be, because some of the issues the court ends up resolving are issues of broad public disagreement."
Like her decisions or not, O'Connor has earned the grudging respect of most people on both sides of the fights.
"She was a public servant who took her duties very seriously," Rosen said, "so even though she never gave everyone everything that they wanted, I'm sure that on reflection she will be missed."
Following the announcement of her retirement, her fellow justices issued statements praising her for the part she played in the institution.
"She is a long-time friend and valued colleague," Rehnquist said.
Scalia called O'Connor a "star" and said she "enhanced the national and international reputation of the Court, and has established -- to the point where it now goes almost without notice -- the role of her sex in the administration of justice."
O'Connor minimized the influence her gender plays in her decision-making.
"A wise old man and a wise old woman reach the same conclusion," she said in 1992. "This should be our aspiration -- that whatever our gender or background, we all may become wise."
She said despite the ideological divides on the high court, she always got along with her fellow justices.
"When you work in a small group of that size, you have to get along," she told ABC News' George Stephanopoulos in 2003. "And so you're not going to let some harsh language and some dissenting opinion affect a personal relationship. You can't do that."
And what did she think the other justices would say about her?
"Oh, I don't know," she said on "Good Morning America." "I don't know what they'd say. I hope they'd say, 'She's a friend and a good colleague.' "
ABC News' Cynthia McFadden, Ellen Davis, Larry Shaw, Christine Gibadlo and Michael S. James contributed to this report.