In 1991, Judge Samuel A. Alito Jr. voted to uphold a Pennsylvania statute that would have required at least some married women to notify their husbands before getting an abortion; a year later, Justice Sandra Day O'Connor cast a decisive fifth vote at the Supreme Court to strike it down.

In 2000, Alito ruled that a federal law requiring time off for family and medical emergencies could not be used to sue state employers for damages; three years later, O'Connor was part of a Supreme Court majority that said it could.

And last year, Alito upheld the death sentence of a convicted Pennsylvania murderer, ruling that his defense lawyers had performed up to the constitutionally required minimum standard. When the case reached the Supreme Court, O'Connor cast a fifth vote to reverse Alito.

The record is clear: On some of the most contentious issues that came before the high court, Alito has been to the right of the centrist swing voter he would replace. As a result, legal analysts across the spectrum saw the Alito appointment yesterday as a bid by President Bush to tilt the court, currently evenly divided between left and right, in a conservative direction.

O'Connor "has been a moderating voice on critical civil liberties issues ranging from race to religion to reproductive freedom," said Steven R. Shapiro, national legal director of the American Civil Liberties Union. "Judge Alito's nomination . . . therefore calls into question the court's delicate balance that Justice O'Connor has helped to shape and preserve."

"With this nomination, Bush is saying 'Bring it on!' " said John C. Yoo, a former Bush administration Justice Department official. "There is no effort to evade a clash with Senate Democrats. That's why conservatives are so happy."

The differences in judicial philosophy between Alito and O'Connor are not absolute. He has not flatly written that Roe v. Wade, the Supreme Court's 1973 abortion rights ruling, should be overturned -- as have some other conservatives who were thought to be on Bush's list for the court.

Alito struck down a New Jersey law that would have banned the procedure known by opponents as "partial-birth" abortion -- just as O'Connor did. His ruling, following the one O'Connor voted for, said the statute was unconstitutional because it did not include an exception for cases in which the woman's health was at risk.

And despite the disagreement on the Family and Medical Leave Act's applicability to the states, the two appear to share a narrow view of the federal government's power to make national laws under its authority to regulate interstate commerce.

The scholarly Alito earned his conservative reputation not through outspoken opposition to the Supreme Court's jurisprudence -- which was the approach taken by Judge Robert H. Bork in his failed bid for the Supreme Court in 1987. Instead, as a member of the U.S. Court of Appeals for the 3rd Circuit, which includes Pennsylvania, New Jersey, Delaware and the Virgin Islands, he sought to uphold precedent as he saw it through his own conservative filter.

An irony of Alito's appointment as a replacement for O'Connor is that, in several of his most controversial rulings, he was exploring how much conservative running room there might be in sometimes-vague legal standards O'Connor herself had helped articulate on the Supreme Court.

One such standard was O'Connor's notion, set forth in a series of abortion cases during the 1980s, that Roe v. Wade protected the abortion right against any "undue burden" a state might try to place on it.

In the 1991 case, Alito joined two other judges in upholding various abortion regulations the Pennsylvania legislature had adopted. But he was the only member of the panel who thought the law's requirement that married women must notify their husbands before having an abortion was not an "undue burden" as O'Connor had defined the concept.

In an opinion that never called for the overruling of Roe or even spoke negatively of it, Alito said that the spousal-notification law would be all right -- in part because it made an exception for cases of spousal abuse.

However, Alito had guessed wrong about O'Connor's meaning. When the case came to the Supreme Court in 1992, she joined a five-justice majority that reaffirmed Roe and ruled that the spousal notification law constituted an undue burden.

On the Family and Medical Leave Act (FMLA), Alito's 2000 opinion concluded that a Pennsylvania state employee could not sue a state agency for damages for allegedly violating his right to paid sick leave.

Citing Supreme Court opinions written or supported by O'Connor, he said that Congress had lacked the power to abrogate the state's sovereign immunity to suit.

But, once again, this attempted application of precedent did not pan out -- because Alito had taken O'Connor's past rulings in a direction O'Connor herself did not want to go. Three years later, O'Connor reached a different result in a different case that presented the same issue.

She joined a 6 to 3 majority of the court in deciding that the FMLA was an appropriate federal response to gender discrimination, as the states had a history of basing their leave policies on the stereotype that women should stay home to take care of sick family members or newborn children.

In the death penalty case, a death row inmate in Pennsylvania argued that his lawyers had failed to investigate possible evidence that might have persuaded the jury not to sentence him to die. But Alito, citing a 1984 Supreme Court opinion by O'Connor, ruled that the lawyers had done a reasonable job, which, he said is all the Constitution requires.

O'Connor cast a fifth vote at the Supreme Court to overrule Alito, joining the four most liberal justices in concluding that the man's trial counsel had failed to investigate his case aggressively enough.

In addition to the late-term abortion case, Alito and O'Connor may agree about the limits to the federal government's power to legislate under the Constitution's commerce clause.

In 1995, O'Connor was part of a five-justice majority that struck down a federal ban on gun possession within 1,000 feet of a school. The court concluded that such a law intruded on local authority without any proof of an effect on interstate commerce.

The next year, Alito, citing that Supreme Court case, dissented from a 3rd Circuit decision that upheld a federal ban on possession of an automatic weapon, arguing that the sale of two guns by one person within Pennsylvania had no wider interstate impact. The Supreme Court declined to consider the gun seller's appeal.

In 1999, Alito upheld a Jersey City holiday display on public property that included a menorah and a creche, as well as Frosty the Snowman, Kwanzaa symbols and a sign explaining the city's intent to celebrate cultural diversity. Alito cited 1984 and 1989 Supreme Court decisions, joined by O'Connor, that had established a rule that such a mixed exhibition, which did not "endorse" a particular religion, would be constitutional.

The Supreme Court declined to review Alito's ruling.

The Supreme Court justices sat for an official portrait yesterday. O'Connor, seated second from right, is considered a moderate swing voter on a court otherwise evenly divided between conservatives and liberals.

Samuel A. Alito Jr. of the U.S. Court of Appeals for the 3rd Circuit has been nominated to replace retiring Supreme Court Justice Sandra Day O'Connor.