WASHINGTON (CNN) -- Supreme Court Justice Sandra Day O'Connor, the first woman to serve on the high court and the key swing vote in some of the nation's highest-profile cases, announced her resignation Friday.

In a letter to the White House, the moderate conservative, said she will step down when her successor is confirmed.

President Bush said O'Connor was well respected.

"Throughout her tenure she has been a discerning and conscientious judge, and a public servant of complete integrity," Bush said. "Justice O'Connor's great intellect, wisdom and personal decency have won her the esteem of her colleagues and our country."

O'Connor, 75, was appointed by President Reagan in 1981 and is considered a moderate conservative on the high court and often cast the pivotal swing vote in important cases.

She dismissed the label, telling CNN recently, "That's something the media has devised as a means of writing about the court, and I don't think that has a lot of validity."

The list of cases in which her vote made the difference is long and notable: limiting affirmative action (Adarand 1995 case requiring "compelling" government interest), permitting public aid and vouchers to religious and parochial schools (Agostini and 2002 Zelman/Cleveland cases), and in several abortion-related cases, where a woman's reproductive rights were narrowly re-affirmed (Casey, Cathcart cases).

A brief statement from O'Connor said she needs to spend more time with her husband, attorney John J. O'Connor, who has been suffering from early stages of Alzheimer's.

This is the first Supreme Court vacancy since 1994, when President Clinton nominated Stephen Breyer, who easily won confirmation.

The Bush White House has held secret meetings on possible replacements and senior officials have interviewed some candidates.

On Friday, Bush said he is looking for candidates "who meet a high standard of legal ability, judgment and integrity, and who will faithfully interpret the Constitution and laws of our country."

Sen. Charles Schumer, D-New York, said: "We would expect the president to maintain the critical balance of the court that Justice O'Connor fought so long and hard for by nominating a consensus, mainstream nominee.

"We should replace Justice O'Connor with a consensus candidate, not an ideologue."

Bush said he will select his nominee in time for the next Supreme Court session.

Some of the people mentioned by legal analysts as possible replacements include, Judge Michael Luttig, 4th U.S. Circuit Court of Appeals, a former D.C. attorney and assistant attorney general; and Judge Harvie Wilkinson, 4th Circuit, appellate court judge in Philadelphia.

O'Connor's move did not come as a big surprise to court watchers and portends a tremendous political battle. Already, conservative and liberal groups have been pushing the White House to fill a potential vacancy with someone who leans toward their points of view.

Her stance on abortion rights has previously drawn criticism from staunchly conservative groups. She served as the swing vote in several abortion-related cases in which abortion rights were narrowly re-affirmed. O'Connor did not back down from her insistence that states place "no undue burden" on the right to an abortion.

There also has been speculation about whether Chief Justice William H. Rehnquist, 80, will step down. The chief justice has thyroid cancer and uses trachea tube to help him breathe.

The Supreme Court ended its term Monday.

24 years of service

O'Connor was surprised when President Reagan fulfilled a campaign pledge to nominate a woman to the high court and chose her in July 1981 to replace Justice Potter Stewart.

Reagan described her at the time as "truly a person for all seasons, possessing qualities of temperament, fairness, intellectual capacity and devotion to public good."

During Senate confirmation hearings in 1981, she was asked about the legacy she hoped to leave.

"Ah, the tombstone question," O'Connor replied. "I hope it says, 'Here lies a good judge.' "

On September 22, 1981, she was unanimously confirmed by the Senate.

In court, O'Connor's demeanor is serious, studied, her questions spare and pointed on the practical effect of laws.

Her toughness and her dry wit were on display when O'Connor was diagnosed in 1988 with breast cancer. She was back on the bench within weeks after treatment.