Justice Sandra Day O'Connor, after seven months on the Supreme Court, is proving a reliable member of the court's conservative bloc and a leader in the campaign to restrain federal judicial power.

In her opinions, she has spoken out against "federal intrusions" in the criminal justice systems of the states and the "absence of finality" in punishing criminal offenders.

She has voted to cut back the right of citizens' groups to challenge government aid to religion in the U.S. courts; to keep federal courts from interfering with state procedures for removing children from parents who abuse them; to make it harder for blacks and women to challenge discriminatory seniority systems in the courts, and to allow states to deny jobs as parole officers to non-citizens.

She supported counties and municipalities in their unsuccessful effort to win exemption from federal antitrust laws for the regulation of cable television, and religious organizations seeking the right to hold prayers on public college campuses.

Court analysts caution that it is early to draw too many conclusions, especially since she has yet to vote publicly on abortion and busing issues currently confronting the court.

But an analysis of the 58 opinions in which she has voted, covering a wide range of the subject matter the court handles each year, suggests she usually conforms with the judicial views of the president who nominated her, Ronald Reagan, and with those of the court's most conservative member, William H. Rehnquist.

Of the 58 cases, 21 have split the court along its ideological lines, with Rehnquist (and usually Chief Justice Warren E. Burger) on one side and William J. Brennan Jr. and Thurgood Marshall on the other.

O'Connor has voted with Rehnquist in 18 of those contests and with Brennan in three.

She has sided most consistently with the conservative wing on criminal cases, voting against the claims of defendants. The two most controversial rulings she has written trimmed the ability of defendants to use the federal courts to challenge their convictions through habeas corpus petitions.

One of her three departures from those allies, however, was also in an important criminal case concerning the death sentence. In that case, she indicated agreement with the tradition of careful supervision of capital punishment laws and their application by the court.

The overall thrust of her voting record is no surprise to the administration that nominated her. She was chosen in part because, as a state appellate judge in Arizona, she was a champion of restoring judicial power to the state courts and a critic of allowing endless opportunities for contesting criminal convictions.

Burger, who shares her views, has given her unusual opportunities to translate her philosophy into law by assigning her to write the majority opinions in several important criminal cases.

In one of these, Engle vs. Isaac, O'Connor produced what amounted to a manifesto against federal "intrusion" into the state criminal justice process. The opinion, a ruling on three criminal cases, stemmed from attempts by three Ohio inmates to have their convictions thrown out because of defects in the jury instructions at their trials.

They had already been tried, had not objected to the jury instructions at their trials, and had been through the conventional process of appealing their convictions.

Then, following the standard route of thousands of state prisoners each year, they turned for relief to the federal courts and another form of appeal, the writ of habeas corpus, a procedure for challenging the legality of imprisonment by finding a defect in arrest or trial procedures. They won in the U.S. Court of Appeals for the 6th Circuit. It was just the sort of victory that O'Connor, Burger and the Reagan administration, in its own law-and-order proposals, have been attacking.

"During the last two decades," O'Connor wrote in the opinion April 5, "our constitutional jurisprudence has recognized numerous new rights for criminal defendants. Although some habeas writs correct violations of long-established constitutional rights, others vindicate more novel claims.

"State courts are understandably frustrated when they faithfully apply existing constitutional law only to have a federal court discover . . . new constitutional commands. In an individual case, the significance of this frustration may pale beside the need to remedy a constitutional violation.

"Over the long term, however, federal intrusions may seriously undermine the morale of our state judges."

The states, she wrote, "possess primary authority for defining and enforcing the criminal law . . . . Federal intrusions into state criminal trials frustrate both the states' sovereign power to punish offenders and their good-faith attempts to honor constitutional rights."

Beyond that, she said, the writs "frequently cost society the right to punish admitted offenders. Passage of time, erosion of memory and dispersion of witnesses may render retrial difficult, even impossible. While a habeas writ may, in theory, entitle the defendant only to retrial, in practice it may reward the accused with complete freedom from prosecution."

The justices reversed the court of appeals, saying that the three defendants, having not raised their objections at trial, had shown insufficient justification for considering them now.

O'Connor won seven votes for the reversal. But she barely held a majority on the reasons, the rhetoric, explaining the judgment. Justices Harry A. Blackmun and John Paul Stevens concurred without joining O'Connor's opinion. Brennan, joined by Marshall, filed a vigorous dissent, accusing O'Connor of a "disregard" for court precedent and calling her views a "conspicuous exercise in judicial activism . . . . "

That was one of seven published opinions by O'Connor. Two of the others, both with divided courts, also involved prisoner petitions. The remaining four were relatively uncontroversial.

On the bench, O'Connor's performance has been highly rated by lawyers arguing before the justices. She appears extremely well prepared, and displays a talent for cutting crisply through legal jargon to the heart of the matter.

Several justices questioned an unusual out-of-court agreement between former president Richard M. Nixon and A. Ernest Fitzgerald, the onetime Pentagon whistleblower, when their case was argued before the court Nov. 30. The question before the court is whether Fitzgerald, who sued Nixon for damages after losing his government job, can take the former president to trial or whether presidents are immune from such damages suits.

Before the arguments, however, Fitzgerald agreed to drop his suit regardless of the court's ruling in exchange for a payment from Nixon. Nixon also agreed to pay Fitzgerald an additional $28,000 if the court sides with the former Pentagon official.

The entire court appeared concerned by this arrangement--which might have rendered the case meaningless--but it was O'Connor who found the words to describe it.

"It's almost a wager on how this court will rule, isn't it?" she asked Nixon's lawyer, Herbert J. Miller.

"I wouldn't so characterize it," he responded sheepishly.