President Reagan, in a break with 191 years of tradition, yesterday nominated Judge Sandra Day O'Connor of Arizona to be the first woman to serve on the U.S. Supreme Court.

The nomination, in fulfillment of a campaign promise to name a woman to one of the first vacancies on the nation's highest court, was hailed by women's groups -- unaccustomed praise from a sector that until now has found little to command in the new administration. But it also drew immediate fire from some of the president's conservative allies, including the politically potent Moral Majority, who say that O'Connor has taken stands favoring abortion and the Equal Rights Admendment.

Calling O'Connor "a person for all seasons," Reagan brushed aside the criticisms of anti-abortion groups, some of which plan a campaign against her confirmation.

"I am completely satisfied with her," a smiling Reagan said as he announced her appointment in the White House press briefing room. Early reaction from the Senate indicated that her confirmation would be easy.

Referring to his campaign pledge to make one of his first Supreme Court appointments "the most qualified woman I could possibly find," the president said he was not appointing O'Connor because of her sex alone.

"That would not be fair to women, nor to future generations of all Americans whose lives are so deeply affected by the decisions of the court," Reagan said. "Rather, I pledged to appoint a woman who meets the very high standards I demand of all court appointees. I have identified such a person."

In a news conference in Phoenix, O'Connor said she was "extremely happy and honored" to have been nominated, but she turned aside substantive questions -- in deference, she said, to the confirmation process.

The administration began searching for a woman nominee soon after retiring Justice Potter Stewart privately told his friend Vice President Bush in mid-March that he wanted to leave the court. Attorney General William French Smith also was notified, although Reagan, who was wounded in an assassination attempt March 30, was not informed of Stewart's intention until April 21.

Nonetheless, the early tip from Stewart gave the administration three months to search quietly for a replacement without being subjected to political lobbying or press speculation.

From the outset, the administration was interested only in women candidates. White House deputy press secretary Larry Speakes confirmed yesterday that Reagan talked only to O'Connor, although Smith interviewed several candidates. Another White House official said all of those to whom Smith talked seriously were women.

By the time the Stewart resignation was made public on June 18, the search already had focused on the 51-year-old O'Connor, a member of the Arizona Court of Appeals. She seemed, said one aide who helped in the inquiry, "almost too good to be true."

O'Connor met the main presidential test of being a judicially qualified woman with a conservative record. In addition, her comparative youth promises to give her a long tenure on the court, and her political experience as a former majority leader of the Arizona state Senate is viewed as an added benefit on a high court that has sometimes been conspicuously lacking in collegiality.

The search was conducted by Smith and White House counselor Edwin Meese III. They also consulted with the most judicially experienced member of the Reagan team, Deputy Secretary of State William P. Clark, who had been a Reagan appointee to the California Supreme Court.

All three men had themselves been considered potential candidates for the appointment, and all took themselves out of the running. Their review of O'Connor's opinions plus an interview with her convinced the president that he had found the woman he was looking for.

Reagan met with O'Connor in the Oval Office on July 1 at a session also attended by Smith, Meese, White House chief of staff James A. Baker III and deputy chief of staff Michael K. Deaver.

Last Monday evening, Reagan telephoned O'Connor, who was back in Phoenix, and told her that the nomination was hers.

In an effort to head off any opposition from single-issue groups, particularly those who interpret the 1980 Republican Party platform as a mandate to name justices who are opposed to abortion, the White House announced at the time it made Stewart's resignation public that no one issue would be used as a "litmus test."

Rather, White House officials said that Reagan would choose a justice who would interpret the law, rather than make new law from the bench. The opposition to O'Connor on the abortion issue is not based on any judicial decision but on her actions as an Arizona state legislator.

In one vote, she opposed a rider on an unrelated bill that would have prohibited free abortions at the University of Arizona hospital. She also reportedly opposed a constitutional amendment that sought to overturn the U.S. Supreme Court's decision liberalizing abortion.

Speakes said yesterday that O'Connor had told the president "she is personally opposed to abortion and that it was especially abhorrent to her. She also feels the subject of the regulation of abortion is a legitimate subject for the legislative area."

The White House had planned to announce O'Connor's nomination later this week. But after The Washington Post reported last week that she was a leading candidate for the nomination, an apposition campaign headed by "right-to-life" groups began pressuring administration officials. And the decision was reached Monday to make the nomination public immediately.

In announcing his decision yesterday morning, the president said of O'Connor: "She is truly a person for all seasons, possessing those unique qualities of temperament, fairness, intellectual capacity and devotion to the public good which have characterized the 101 'brethren' who have preceded her. I commend her to you and urge the Senate's swift bipartisan confirmation so that, as soon as possible, she may take her seat on the court and her place in history."

Reagan was loudly applauded last night during a political fund-raising speech in Chicago when he departed from his text to say that his appointment of O'Connor had made it "a very happy day for me and, I hope, for our country."

"Judge O'Connor, in my view, will bring new luster and new strength to the Supreme Court," the president said. "I feel certain that her term upon the bench will be one of the proudest legacies of my presidency."