ASHINGTON, June 26 — The Supreme Court spared the life of a convicted killer in Maryland today, declaring that his inexperienced lawyers had failed to represent him adequately at his trial 14 years ago.

In a 7-to-2 vote in which they restated the need for a thorough defense in death-penalty cases, the justices threw out the sentence imposed on the convicted man, Kevin Wiggins, a borderline retarded man with no prior record. Mr. Wiggins was convicted of drowning an elderly Baltimore County woman in 1988 in the apartment complex where he worked as a painter and handyman.

At Mr. Wiggins's trial in 1989, his lawyers elected to have him tried by a judge without a jury, and the judge found him guilty. Then a jury deliberated on what sentence to impose.

The court ruled that the defendant's lawyers had failed to adequately convey to the jury the horrendous abuse Mr. Wiggins endured throughout his childhood. It had been from his alcoholic mother, who burned his hands on the stove as punishment and sent him into the street to beg for food, and in various foster homes, where he was subjected to sexual assaults and beatings.

Without setting specific new standards, the court restated a defendant's constitutional right to competent representation in cases in which a life is at stake.

"The mitigating evidence counsel failed to discover and present in this case is powerful," Justice Sandra Day O'Connor wrote. "Given both the nature and extent of the abuse petitioner suffered, we find there to be a reasonable probability that a competent attorney, aware of this history, would have introduced it at sentencing in an admissible form."

Chief Justice William H. Rehnquist and Justices John Paul Stevens, Anthony M. Kennedy, David H. Souter, Ruth Bader Ginsburg and Stephen G. Breyer agreed. The ruling leaves the conviction intact but grants a new sentencing hearing.

Justices Antonin Scalia and Clarence Thomas dissented, with Justice Scalia saying the majority's reasoning ranged "from the incredible up to the feeble" and that it had given far too much weight to "uncorroborated gossip" about childhood abuse.

The verdict and sentence had been upheld by Maryland state courts, then overturned by a Federal District Court judge, who said the case was so shaky that the sentence was "of moral concern." The United States Court of Appeals for the Fourth Circuit, in Richmond, Va., reinstated the conviction and sentence, concluding that Mr. Wiggins had had adequate representation, and that his trial lawyers' conduct amounted to tactical decisions that could not be easily questioned.

The case, Wiggins v. Smith, No. 02-311, has attracted wide attention. Lawyers who represent death row prisoners in appeals have long complained that the defendants, often poor and of limited intelligence, get inadequate help during their trials.

Mr. Wiggins's current lawyer, Donald B. Verrilli Jr. of Washington, said he was "quite happy" with today's decision but would continue to work for his client's exoneration and freedom.

"We're convinced of his innocence," Mr. Verrilli said in a telephone interview from Italy, where he is vacationing.

Mr. Verrilli left open the possibility of seeking a new trial.

Mr. Verrilli called the ruling "a powerful reaffirmation of the right to counsel and the historic importance of habeas corpus." He said his client was "a dupe, essentially, of his girlfriend and her brother."

The elderly victim, Florence Lacs, was found drowned in the bathtub of her ransacked apartment in Baltimore County on Sept. 17, 1988. Five days later, the police found Mr. Wiggins, then 27, and his girlfriend, Geraldine Armstrong, then 42, traveling in the victim's car. The victim's credit cards, used at several stores from Sept. 15 onward, were found in their possession.

Both Mr. Wiggins and his girlfriend were charged with murder and robbery, although the counts against her were dropped. She testified against Mr. Wiggins, saying he had arrived at her home the night of Sept. 15 in Ms. Lacs's car.

Two jailhouse informers testified that the defendant, who had done painting work in the apartment complex where the victim lived, had admitted the killing to them. But they later recanted, and prosecutors considered their testimony so unreliable that they disavowed it.

Ms. Armstrong at first maintained that she was not familiar with the apartment complex. Later, she acknowledged that one of her brothers lived in an apartment on the floor below Ms. Lacs's residence.

And the all-important time of Ms. Lacs's death was never established. A pathologist testified for the defense that Ms. Lacs died no earlier than Sept. 17, meaning that she was still alive when Mr. Wiggins and Ms. Armstrong were found with her car and credit cards. Another pathologist testified for the prosecution that it was impossible to determine the time exactly.

The Justice Department under Attorney General John Ashcroft entered the case on Maryland's side, calling the legal representation that Mr. Wiggins received "eminently reasonable."

But Mr. Ashcroft's predecessor, Janet Reno, filed a brief on behalf of Mr. Wiggins, along with nine other current and former prosecutors. The credibility of the criminal justice system is at stake in cases like this one, they said.