A hurriedly prepared, error-filled memo by a young Justice Department lawyer convinced President Reagan to go through with nominating Judge Sandria O'Connor to the Supreme Court, even at grave political risk.

The memo softened O'Connor's pro-abortion record that has stunned Moral Majority elements in Reagan's coalition. That the president accepted it at face value broadened suspicions that his narrow flow of information subjects him to staff manipulation.

Even so, if the president took seriously the Moral Majority and its issues, he would have found it difficult to pick O'Connor. Thus, fundamentalists who turned on Jimmy Carter after they felt deceived by him may feel the same way about Ronald Reagan.

O'Connor surely will be confirmed. But important conservative Republicans in Congress, while keeping mum publicly, grumble privately that the president has lost control of his own administration to moderate forces in general and chief of staff James Baker III in particular.

The remarkable fact is that Reagan was unaware that the right-to-life movement found O'Connor totally unacceptable until her probable nomination leaked out just before the Fourth of July weekend. The resulting avalanche of opposition then gave the president serious pause.

For example, Trudy Camping, one of O'Connor's former state Senate colleagues, sent the White House a decade-old stack of clippings about O'Connor. The revaled a moderate social liberal supporting the Equal Rights Amendment for women, advocating free choice on abortion and urging caution in restricting pornography.

On Monday, July 6, the president telephoned Attorney General William French Smith, who had given Reagan the Justice Department's O'Connor recommendation. Reagan wanted a quick check on this abortion business. Smith turned the task over to his young counselor, Kenneth W. Starr, who telephoned O'Connor herself.

The next day, Starr handed Smith a two and one-half page memo giving O'Connor a clean bill of health on abortion by using legal gymnastics to explain her Arizona legilative record. While Starr's memo said O'Connor "has no recollection" of how she voted on a 1970 bill to legalize abortion, in fact she was a co-sponsor of the measure and voted for it as it was defeated 6-to-3 in committee.

"Judge O'Connor further indicated, in response to my questions," Starr concluded his memo, "that she had never been a leader or outspoken advocate on behalf of either pro-life or abortion-rights organizations. She knows well the Arizona leader of the right-to-life movement, a prominent female physician in Phoenix, and has never had any disputes or controversies with her."

Starr did not bother to check with that "prominent female physician" -- Dr. Carolyn Gerster, a national anti-abortion activist. If he had, the attorney general's man would have gotten an earful. Gerster told us "I had an adversary position with Sandra O'Connor" in the 1970s when the Supreme Court nominee was "one of the most powerful pro-abortionists in the [Arizona] Senate." Gerster still harbors an 11-year-odl grievance, claiming Senate Majority Leader O'Connor broke her word by burying an anti-abortion proposal in caucus.

Based on Starr's memo, Smith reassured Reagan that O'Connor offered no problems. Baker, David Gergen and other senior presidential aides said the same thing, contending only right-wing kooks were making a fuss. Reagan agreed, telephoning prominent anti-abortion Republicans to reassure them that "she's all right."

Eager to annouce the nomination before opposition could build, nobody at the White House bothered to probe O'Connor's record. But right-wingers will bother, not in realistic hope of blocking her nomination, but to deter Reagan from similar choices for future court vacancies.

No matter how pure future Reagan justices are, however, innocence has departed for right-to-life activists. Dr. Gerster cannot forget a 45-minute meeting with Reagan in Rye, N.Y., on Jan. 17, 1980, in which candidate Reagan promised her that his first appointment to the court would share their anti-abortion views. She chooses to believe that the president has been misled by advisers.

But the more plausible explanation is that Reagan shares the view of Jim Baker and his other aides that the Moral Majority is not vital to his political coalition. He has given that signal by ignoring its sensibilities in selecting Sandra O'Connor.