ASHINGTON, Dec. 13 - The Supreme Court ruled on Monday that faced with overwhelming evidence that a client is guilty of capital murder, a defense lawyer can make a reasonable strategic decision to concede guilt in open court, even if the client has not authorized such a strategy, in order to preserve some credibility with the jury that will soon decide whether to impose a death sentence.

The 8-to-0 decision, with an opinion by Justice Ruth Bader Ginsburg and with Chief Justice William H. Rehnquist not participating, overturned a ruling by the Florida Supreme Court that a lawyer who concedes a defendant's guilt, for whatever reason, in the absence of explicit authorization, has deprived the client of the effective assistance of counsel.

Applying that rule, the Florida Supreme Court last year ordered a new trial for Joe Elton Nixon, who was convicted and sentenced to death in 1984 for kidnapping a woman, tying her to a tree, and setting her on fire while she was still alive. He then confessed the killing to his brother, took two of his victim's rings to a pawnshop and tried to sell the sports car he stole from her.

Justice Ginsburg said that Mr. Nixon's lawyer, Michael Corin, who was experienced in defending murder cases, calculated that a denial of guilt in the face of "overwhelming evidence" of the gruesome crime would have appeared incredible to the jurors before whom he would soon be asking for leniency in the sentencing phase. The lawyer's strategy was aimed at "preserving his credibility," she said, adding, "In a capital case, counsel must consider in conjunction both the guilt and penalty phases in determining how best to proceed."

That the strategy failed - the jury decided on a sentence of death after only three hours of deliberation - did not necessarily mean that the representation was ineffective, Justice Ginsburg said. She added that while lawyers ordinarily had a duty to consult with their clients on questions of "overarching defense strategy," that obligation did not extend to "every tactical decision."

In this instance, the client was unresponsive, neither giving nor withholding his consent, when the lawyer tried three times to discuss the trial strategy with him. Mr. Corin had previously tried to negotiate a plea agreement with the prosecution, offering a guilty plea in exchange for a sentence of life in prison, but the effort failed because the state would accept nothing less than a death sentence.

Under the opinion, Florida v. Nixon, No. 03-931, the Florida court must now evaluate Mr. Corin's courtroom behavior under a standard more flexible than the absolute rule it applied. The two-part inquiry for ineffective assistance of counsel, which the Supreme Court first set out in a 1984 decision, Strickland v. Washington, asks whether the lawyer's performance was deficient and, if so, whether the deficiency actually hurt the defense.

The answer to both those questions is almost certainly "no." At the end of the penalty phase of the trial, the trial judge commended Mr. Corin for his "excellent analysis of the reality of his case." At the penalty phase, the lawyer tried to persuade the jury that Mr. Nixon's low I.Q., possible brain damage and history of mental instability should all militate against a death sentence.

Under Supreme Court precedents, a defendant must give explicit consent to a guilty plea. Justice Ginsburg said on Monday that the Florida Supreme Court's mistake in this case was to equate the concession the lawyer made in court with a formal plea of guilty. While a guilty plea forfeits the right to a trial and relieves the prosecution of the need to prove its case, she said, in this case Mr. Nixon retained all ordinary trial rights and the government was still obliged to put on evidence. Consequently, Justice Ginsburg said, the Florida court's analogy was incorrect.

The case was argued on Nov. 2, shortly after Chief Justice Rehnquist began aggressive treatment for thyroid cancer. While the court first announced that the chief justice, who has been absent since Oct. 22, would participate in all the court's cases by reading the briefs and argument transcripts, that turned out not to have been the case, at least for the cases the court heard in November.

Kathleen Arberg, the court's public information officer, said on Monday that the chief justice had decided to participate only in those November cases that would otherwise have resulted in a 4-to-4 tie. He will participate in all the cases argued in the court's December sitting, she said.

Despite the announcement on Friday that Chief Justice Rehnquist planned to administer the oath of office at President Bush's second inauguration, on Jan. 20, there has been no indication whether he will be back in court in time for the next argument session, which begins Jan. 10.

The brief session the court held on Monday morning to announce decisions - there were no arguments - found Justice Antonin Scalia in the unaccustomed role of presiding over the proceedings, evidently for the first time in his 18 years on the court. The three justices with greater seniority - Chief Justice Rehnquist and Justices John Paul Stevens and Sandra Day O'Connor - were all absent.

Justice Scalia wrote the 8-to-0 opinion for the court in another criminal-law decision the court issued on Monday. The question in Devenpeck v. Alford, No. 03-710, was whether an arrest for which a police officer gives an improper reason is therefore invalid, even if there are justifiable reasons for the arrest.

In this case, two Washington State troopers stopped a driver on suspicion of impersonating a police officer. When the troopers noticed that the man, Jerome A. Alford, was recording their encounter on a tape recorder, they told him that he was under arrest for violating the state's Privacy Act, which makes it a crime to record a conversation without the other person's consent.

Mr. Alford objected that the Washington Court of Appeals had ruled that the law did not apply to conversations with police officers. He was correct, and the charge was dropped. He then sued the officers in federal court for violating his constitutional rights by arresting and imprisoning him without probable cause.

In their defense, the officers asserted that they actually had enough evidence to have arrested him for impersonating an officer. A jury ruled for the officers, but the United States Court of Appeals for the Ninth Circuit rejected that verdict as unsupported by the evidence. The impersonation offense was not "closely related" to the stated reason for the arrest, the court said.

In his opinion overturning the appeals court, Justice Scalia said an arresting officer's subjective state of mind is irrelevant. "Whether probable cause exists depends upon the reasonable conclusion to be drawn from the facts known to the arresting officer at the time of the arrest," he said; there is no requirement that a valid reason be "closely related" to the stated reason. The court returned the case to the Ninth Circuit for a new assessment on whether probable cause existed.