Sandra D. O'Connor received a qualified endorsement from the American Bar Association yesterday as the Senate began the confirmation process for the first woman Supreme Court nominee.

The ABA panel, which rates judicial nominees, told the Senate Judiciary Committee that O'Connor met "the highest standards of judicial temperament and integrity." But it questioned her limited experience as a judge and practicing lawyer, saying that it "has not been as extensive or challenging as that of some other persons who might be available for appointment."

The rating of the ABA's Committee on the Federal Judiciary "could have been a bit" more favorable, one administration source acknowledged yesterday. But it was thought to be favorable enough not to pose any obstacle to a smooth confirmation and a swearing-in of O'Connor within three weeks.

The ABA committee sent its findings yesterday to the Judiciary Committee, which begins three days of hearings today on O'Connor. Questions from conservatives about abortion and her position on the Equal Rights Amendment are expected to produce the only wrinkles in the process.

Traditionally, an unfavorable ABA rating can severely damage a nominee's confirmation chances. The rating of O'Connor was, on the whole, favorable though it contrasted with the ABA rating of the last nominee, John Paul Stevens. The ABA said he met "the highest standards" of professional competence.

O'Connor, the committee said in its letter yesterday, "has the professional qualifications required of an associate justice of the Supreme Court of the United States."

O'Connor has served on the Arizona Court of Appeals for 18 months and has no federal judicial experience. The ABA committee said that made it difficult to "form a judgment on whether she meets the highest standards" of competence. Therefore the committee abandoned the rating system it used for Stevens, by which it rated nominees "well qualified, qualified, not opposed" or worse, and simply provided the senators with an overall judgment on O'Connor, based on extensive interviews with judges, lawyers and others who are familiar with her record.

"Those who have worked with Judge O'Connor," the seven-page ABA report said, "describe her as very intelligent, analytical, thorough and hard-working. The diversity of her experience as a practicing lawyer, legislator and judge provides a valuable background for a Supreme Court justice. She is dedicated to the legal profession and has made significant contributions to her community."

A panel of lawyers and professors that examined her judicial opinions "expressed differing views concerning the strength of her opinion writing," the report said, concluding that they were "competently written" in a "clear and logical" writing style.

Reagan administration officials acknowledged privately that the rating, as one put it, "could have been a bit better." It also could have been worse, they suggested, considering the traditional notion of the organized bar that a Supreme Court nominee should have extensive federal court experience as part of his or her qualifications.

The Judiciary Committee hearings, which will be broadcast live on public television and radio, are expected to last three days. O'Connor's testimony and her answers to questions on judicial philosophy will probably consume the first two days.

O'Connor has spent weeks reading briefing papers on Supreme Court history and past rulings in preparation for the hearings. During the past two days, Justice Department officials have rehearsed the hearings with her, allowing her to formulate answers to potential questions senators might ask.

On Friday, about 25 witnesses, including several anti-abortion leaders and numerous O'Connor supporters from Arizona, will testify. Committee consideration of the nomination is tentatively scheduled for next Tuesday and the nomination could go to the Senate that day.

Justice Department officials have targeted Sept. 24 or 25 as the swearing-in date, demonstrating their confidence in an easy confirmation.

O'Connor apparently shares their confidence and has invited many of her friends to Washington for a celebration at that time, sources said.