When President Ronald Reagan nominated Justice Sandra Day O'Connor, then a relatively unknown Arizona state judge, to sit on the nation's highest court, antiabortion groups opposed her, fearing she was not fully on their side.

Yesterday, O'Connor seems to have proven them right. She voted with the court's liberal wing to strike down a portion of an antiabortion law that would have required a minor, virtually without exception, to notify both parents before obtaining an abortion.

With that vote O'Connor clearly broke ranks for the first time with the three other Reagan-appointed justices: Chief Justice William H. Rehnquist and Justices Antonin Scalia and Anthony M. Kennedy.

The three other Reagan appointees and Justice Byron R. White agreed with Minnesota's argument that there was no constitutional need to give minors the option of seeking a judge's permission to avoid the requirement that both parents be notified of the intention to obtain an abortion -- O'Connor's position.

But O'Connor then demonstrated once again that she is not prepared to climb off the fence she has long straddled on abortion issues. While supporting the liberal majority on key aspects of yesterday's case, her concurring opinion, in an argument only she offered, also said that two-parent notification was permissible -- as the conservatives had argued -- as long it was accompanied by a judicial waiver option.

Last term, in an abortion case from Missouri, O'Connor declined to join her conservative colleagues in an opinion that effectively would have undone Roe v. Wade, the 1973 decision that legalized abortion.

Yet she did so in a way that left her the option of voting to overturn Roe in some future, possibly more appropriate, case. "There will be time enough to reexamine Roe. And to do so carefully," O'Connor said.

The Minnesota case pushed her closer to confronting the issue and, coupled with her vote in the Missouri case, indicated to several analysts that Roe is unlikely to be overturned in the absence of another conservative appointment to the court.

"I don't think as of now there are five votes to overturn" Roe, said Gary Bauer, a former Reagan White House domestic policy adviser. "I would assume folks at the White House are going to get real nervous the first time they have a vacancy they have to deal with."

O'Connor's vote did not prompt victory cries from abortion-rights groups, however, because her brief concurrence gave little indication of her position on a number of larger abortion questions, and gave her substantial leeway for future decisions.

"She managed once again to say virtually nothing," said Duke University Law School Prof. Walter Dellinger.

In a 1983 case, O'Connor said she would uphold state antiabortion laws unless they were "unduly burdensome." Since then, she has approved all state restrictions before her, leading both sides to wonder whether any abortion law would fail O'Connor's undue burden test.

In yesterday's Minnesota case, O'Connor noted approvingly that the high court had allowed judicial bypass provisions to "avoid unduly burdening the minor's limited right to obtain an abortion."

But her opinion "did not find {the Minnesota law} unduly burdensome," Dellinger said, only that it was irrational.