WASHINGTON — Justice Sandra Day O'Connor, the first woman on the Supreme Court and its decisive voice on such critical issues as abortion, affirmative action and religion, announced her resignation yesterday, touching off a fierce battle over her replacement.

Her surprise decision to retire after 24 years on the high court was a political thunderbolt because of her role in moderating the court's conservative wing. A staunchly conservative replacement could tip the court's balance on a variety of topics, including abortion rights, gay rights, civil liberties, the death penalty, racial preferences and the government's regulatory powers.

On Capitol Hill, the Senate braced for the possibility of a bitter, divisive confirmation debate that could sidetrack other legislative business for months.

O'Connor's resignation creates the first Supreme Court vacancy in 11 years, and the confirmation process will play out in the relatively new world of 24-hour news, Internet interest groups, ideologically driven bloggers and polarized politics.

O'Connor, 75, said she was stepping down to spend more time with her husband, John, who has Alzheimer's disease.

"It has been a great privilege, indeed, to have served as a member of the court for 24 terms," she said in a note delivered to President Bush yesterday morning. "I will leave it with enormous respect for the integrity of the court and its role under our constitutional structure."

Bush praised the departing justice and promised to name a replacement in time for the court's next term, which begins in October. White House aides said he wouldn't announce his nomination until after Friday, when he returns from a trip to Europe.

The list of possible candidates includes Attorney General Alberto Gonzales, who would be the first Hispanic on the court. The most likely female nominees appear to be appellate court Judges Edith Jones and Edith Brown Clement, both on the 5th U.S. Circuit Court of Appeals in New Orleans.

Other appellate judges who could be tapped for promotion are J. Harvie Wilkinson III of the 4th Circuit in Richmond, Va.; J. Michael Luttig, also of the 4th Circuit; Michael McConnell of the 10th Circuit in Denver; Samuel Alito of the 3rd Circuit in Philadelphia; Emilio Garza of the 5th Circuit; and John Roberts of the U.S. Court of Appeals for the District of Columbia.

After phoning O'Connor yesterday, Bush said he first wanted to praise the Arizona cowgirl who rose to become "one of the most admired Americans of our time. ... "

Bush said he "will be deliberate and thorough" in picking her successor. "The nation deserves, and I will select, a Supreme Court justice that Americans can be proud of," Bush said. He said he hoped his nominee would undergo "a dignified process of confirmation" in the Senate.

The president said he'd consult the Senate before making his choice, but he offered no clues about his intentions with regard to the court's philosophical balance.

O'Connor specified that her resignation wouldn't take effect until her replacement is confirmed.

Her fellow justices paid tribute to her in statements issued by the court.

Chief Justice William Rehnquist called her a "valued colleague," and added, "I shall miss her greatly." Justices Antonin Scalia and Clarence Thomas, two stalwart conservatives who often disagreed with O'Connor, praised her temperament.

Scalia called her "a star" who "shaped the jurisprudence of this court more than any other associate justice." Thomas said she was "civil in dissent and gracious when in the majority," and always "warm and courteous."

Justice Ruth Bader Ginsburg, the court's only other woman, recalled O'Connor's surprise appearance in a 1996 theatrical production of William Shakespeare's "Henry V." O'Connor's character uttered the line "Hap'ly, a woman's voice may do some good."

"Sandra Day O'Connor's voice has done enormous good in the pursuit of justice for all in our land and the world," Ginsburg said.

Activists rally

Political activists swung into action within minutes of her announcement. O'Connor's planned departure gave groups that had been on alert for a possible resignation by Rehnquist a new, even more pressing reason to rally supporters.

"Her replacement will turn the direction of this court," said the Rev. Rob Schenck, the president of Faith and Action, a conservative Christian group. "We are already praying and working for a nominee that will not waffle as she did."

The liberal group MoveOn began airing a new television ad urging Bush to avoid an "extremist" nominee.

"The president should honor O'Connor and appoint a moderate Supreme Court justice," said Ben Brandzel, the organization's advocacy director. "If President Bush nominates an extremist, it will be up to senators to say no, and the American people will make sure that they do just that."

Conservative allies of the president responded with their own media onslaught, including a webcast mocking Democrats in a satirical news show by reporting that they were opposing George Washington and Benjamin Franklin after Bush named them to the court.

Sen. Edward Kennedy, D-Mass., whose early opposition to Robert Bork helped doom his nomination to the Supreme Court in 1987, threatened to do the same to a Bush candidate. "If the president abuses his power and nominates someone who threatens to roll back the rights and freedoms of the American people," Kennedy said, "then the American people will insist that we oppose that nominee, and we intend to do so."

Sen. Arlen Specter, R-Pa., the chairman of the Senate Judiciary Committee, said: "This is a very polarized country when it comes to the issues which will come to the Supreme Court. I would expect people to want their choice. And everybody can't have their own way."

Specter said he expected to hold confirmation hearings in September but didn't rule out action in August.

A resignation announcement from Rehnquist would roil the waters even more, but that doesn't seem as likely as it did earlier this year. The 80-year-old chief justice has thyroid cancer but seems to have rallied recently.

Democrats said they'd consider a filibuster, a parliamentary tactic that involves unlimited debate, to block any nominee they considered extremist. Although Senate confirmation requires a bare majority — 51 votes in the 100-member Senate — it takes 60 votes to end a filibuster. In a deal negotiated with Republicans in May, centrist Democrats had said they'd use the filibuster against judicial nominees only under "extraordinary circumstances."

"That's an option," Sen. Christopher Dodd, D-Conn., said when asked yesterday about filibustering. "I hope we can avoid that."

Women's changing role

O'Connor's decision to retire capped a remarkable career that mirrored the changing role of women in U.S. life. After graduating third in her class from Stanford University law school in 1950 — two notches behind classmate Rehnquist — the only job offer she received was a chance to become a legal secretary.

Three decades later, President Reagan named her to the nation's highest court. The appointment, approved in a unanimous Senate vote, transformed one of the last all-male bastions in government and made O'Connor a reluctant feminist icon.

"I think that I bring to the court differences in background that are more germane than my gender," she told Ladies' Home Journal in 1982. "I think the important fact about my appointment is not that I will decide cases as a woman, but that I am a woman who will get to decide cases."

For the past 24 years, she has decided the Supreme Court's biggest cases. She has been neither a predictable conservative nor a true liberal. Rather, she set out to redefine the middle. In doing so, she became, in the eyes of many, the court's most powerful justice.

She nearly single-handedly preserved the constitutional right to abortion, even though she was an early critic of the Roe v. Wade ruling that legalized it. She believed the court made a mistake in the 1970s by taking on the issue, and she said states should have more authority to regulate abortion. In 1989, however, she declined to go along with overturning Roe v. Wade in a pending Missouri case. She sided with the court's conservative wing in the ruling that decided the 2000 election for Bush and was a strong advocate of states' rights.

She was diagnosed with breast cancer in 1988, but her swift return to the bench after treatment helped quiet resignation rumors then.

After her telephone conversation with the president yesterday morning, O'Connor left Washington for a vacation in Wisconsin. She did not give interviews.

In his phone call to O'Connor yesterday morning, Bush harked back to her childhood, during what aides called an emotional conversation for both. "For an old ranch girl, you turned out pretty good," the president told her.

Material from The Washington Post, Los Angeles Times and Chicago Tribune is included in this report.