The court has been closely divided in death row cases, with Justice Sandra Day O'Connor often in the middle.
President Bush's choice to replace her, John Roberts, has a limited track record. Roberts, 50, showed little sympathy for prisoner appeals as a government lawyer in the Reagan administration, but later did free legal work for a death row inmate.
In a February 1983 memo while serving in the Reagan White House, Roberts suggested that the high court could cut its caseload by "abdicating the role of fourth or fifth guesser in death penalty cases."
Stevens used a weekend speech to the American Bar Association to underscore the matter's prominence at the court, noting evidence of "serious flaws."
His remarks provide the first sign of internal dismay over the retirement of O'Connor, a 75-year-old pragmatist who has been a key voter in affirmative action, abortion rights and the death penalty.
So far, much of the focus of the Roberts nomination has been on matters like abortion and civil rights - not the death penalty. His Senate confirmation hearings begin Sept. 6.
"It doesn't appear to be shaping up as a major issue," said Kent Scheidegger, legal director of the Criminal Justice Legal Foundation, a pro-death penalty group.
Scheidegger said that although Roberts' wife, Jane, is a member of a group that opposes capital punishment, Roberts has had no opportunities to vote on death cases in his two years on a federal appeals court.
Stevens has evolved into the Supreme Court's most liberal member, in the 30 years since his nomination by Republican President Ford.
Another justice who made a similar transformation was Harry A. Blackmun, who months before his 1994 retirement declared: "I no longer shall tinker with the machinery of death."
Stevens, who at 85 is the oldest justice, has given no hints that he will retire soon. A Stevens retirement while Republicans control the White House and Senate would likely dramatically reshape the court.
Stevens, speaking in his hometown of Chicago, started his Saturday night speech to lawyers with what he called the "sad news" that O'Connor was leaving the court.
"It's really a very, very wrenching experience," he said.
The comments were not part of his prepared remarks for the evening, to honor liberal former judge and congressman Abner Mikva, nor were the remarks about capital punishment.
Stevens said DNA evidence has shown "that a substantial number of death sentences have been imposed erroneously."
"It indicates that there must be serious flaws in our administration of criminal justice," he said.
Death penalty cases dominate the work of the high court. Week after week justices deal with final emergency appeals, sometimes filed in the late night hours.
In their last term which ended in June, justices overturned the death sentences of four inmates, ruled that states cannot put to death killers who were not at least 18 years old at the time of the crime and held that it was unconstitutional to force defendants to appear before juries in chains during a trial's penalty phase.
Justices already have four capital cases on their docket when they return to work in October, including a potentially significant issue of letting inmates have a new chance to prove their innocence with DNA evidence.
Other Supreme Court justices, including O'Connor and Ruth Bader Ginsburg, have also spoken out about concerns that defendants in murder cases are not adequately represented at trial.
But Stevens told the ABA that the problems were more dramatic.
He said the jury selection process and the fact that many trial judges are elected work against accused murderers. He also said that jurors might be improperly swayed by victim-impact statements.
Stevens was speaking in Illinois, his home state and a place that has been roiled by controversy over the death penalty. In 2000, wrongful convictions led then-Gov. George Ryan to halt all executions.
It also came just a day after a Virginia jury decided against the inmate whose case led to a 2002 Supreme Court ban on executing the mentally retarded. The jury said Daryl Atkins was mentally competent and could be put to death.
Stevens wrote that 2002 Atkins decision, which was joined by O'Connor. One of the three dissenters was Chief Justice William H. Rehnquist, who hired Roberts as a law clerk in 1980. A year later, as a Justice Department lawyer, Roberts wrote in a memo that the availability of federal court appeals, "particularly for state prisoners, goes far to making a mockery of the entire criminal justice system."
Stevens, however, laid out the case for close review of appeals, pointing to "special risks of unfairness" in capital punishment.
According to the anti-capital punishment Death Penalty Information Center, more than three dozen death row inmates have been exonerated since 2000.
Said Scheidegger, "I wouldn't say that 20 or 30 cases out of 8,000 constitutes a broken system."