WASHINGTON – The Supreme Court yesterday threw out yet another sentence for a death row inmate, issuing a warning to state courts in a 17-year-old Pennsylvania case that shoddy defense work wouldn’t be tolerated.

The justices have been particularly active in death penalty issues this session, making it unlawful to execute juveniles, scolding prosecutors for stacking a jury on racial lines, and ruling it was unconstitutional to force defendants to appear before juries in chains during a trial’s penalty phase.

“It’s hard to imagine a term that has been more significant in recent years in illustrating the court’s firmness and seriousness in addressing defects in the American capital punishment system,” said Jordan Steiker, a death penalty expert at the University of Texas law school.

Competency of legal counsel is perhaps the biggest unresolved death penalty issue now that the court has said juveniles and the mentally retarded may not be executed.

The competency issue was the centerpiece of the appeal by Ronald Rompilla, who was convicted of robbing, stabbing, and setting on fire a tavern owner in Allentown, Pa., in 1988. Justices sided with him in a 5-4 ruling that gives him a new penalty trial. More broadly, the ruling means defense lawyers will have to more vigorously investigate capital defendants’ past records for evidence that might save their lives.

The ruling was cheered by death penalty opponents for its close scrutiny of ineffective assistance of counsel claims, by far the most commonly used defense by death row inmates. Supporters of capital punishment, meanwhile, sought to minimize its impact.

“This decision will change the outcome in relatively few capital cases,” said Kent Scheidegger, legal director of the Criminal Justice Legal Foundation, who filed a brief supporting prosecutors.

Justice Souter, writing for the majority, said public defenders were wrong when they failed to review records showing mitigating evidence of Rompilla’s possible mental illness and traumatic upbringing, even after prosecutors warned that they planned to use the documents against him.

“It flouts prudence to deny that a defense lawyer should try to look at a file he knows the prosecution will cull for aggravating evidence, let alone when the file is sitting in the trial courthouse, open for the asking,” Justice Souter wrote, citing in part a violation of American Bar Association standards on lawyer competency.

He was joined by other members of the court’s more liberal wing – Justices Stevens, Ginsburg, and Breyer – as well as Reagan appointee Justice O’Connor, who provided the swing vote.

In a biting dissent, Justice Kennedy blasted the majority for imposing a new, “rigid requirement” forcing defense attorneys to review voluminous files that prosecutors might possibly use. He said Rompilla’s attorneys had reasonably relied on testimony from mental health experts and family members.

Justice O’Connor filed a separate opinion emphasizing that her conclusion was based on the circumstances of Rompilla’s case and should not be read too broadly. Other competency of legal counsel claims must continue to be decided case by case, she said.