ATLANTA (AP) — A strongly worded dissent in the U.S. Supreme Court's narrow decision this week upholding the use of an execution drug offered a glimmer of hope to death penalty opponents in what they considered otherwise a gloomy ruling. One advocate went so far Tuesday as to call it a blueprint for a fresh attack on the legality of capital punishment itself.

But even those who see Justice Stephen Breyer's dissent as a silver lining think it will take time to mount a viable challenge.

And Breyer's words don't change the fact that the Supreme Court has consistently upheld capital punishment for nearly four decades. The five justices forming the majority in Monday's decision made it clear they feel that states must somehow be able to carry out the death penalty.

In disagreeing with the 5-4 ruling that approved Oklahoma's use of an execution drug, Breyer, joined by Justice Ruth Bader Ginsburg, called it "highly likely that the death penalty violates the Eighth Amendment," which protects against cruel and unusual punishment.

"It was a sweeping and powerful dissent that issues an invitation that we should accept, which is to make the case for why today the death penalty itself is no longer constitutional," said Cassandra Stubbs, director of the Capital Punishment Project of the American Civil Liberties Union.

"Rather than tinker with these questions of how we should kill, we should be asking the more fundamental and the larger question of whether we as society should still be executing anyone at all," she added.

In the case that prompted Monday's opinion, death-row inmates in Oklahoma had objected to the use of the sedative midazolam after the drug was blamed in several botched executions. Their argument was that the drug does not reliably induce a coma-like sleep that would prevent them from experiencing the searing pain of the paralytic and heart-stopping drugs that follow.

Oklahoma, Florida, Ohio and Arizona have used the drug in killing 13 inmates total since the start of 2014, according to the Death Penalty Information Center.

Writing for the majority, Justice Samuel Alito said arguments that the drug could not be used effectively as a sedative in executions were speculative. He dismissed problems in executions in Arizona and Oklahoma as "having little probative value for present purposes."

The Supreme Court struck down capital punishment in 1972, and some justices thought at the time it would be the end of the death penalty. But many states just wrote new laws and the court reinstated it four years later.

Stubbs wouldn't speculate when a new constitutional challenge to the death penalty might make its way to the Supreme Court, but said it's "on the horizon." Already, there has been a trend of decreasing use of the death penalty, she said, citing the falling number of executions and new death sentences issued.

Multiple factors are driving that trend, including increased awareness of exonerations of death row inmates, the costs of bringing capital cases and giving juries the option of life without parole, allowing them to impose a very serious punishment without death.

The fact that more than 100 death row inmates have been exonerated demonstrates that the death penalty is unreliable, Breyer wrote. He also argues that the death penalty is imposed arbitrarily, is no longer used by most of the country and that it takes far too long to carry out, which undermines any deterrent effect.

Those concerns could provide guidance to capital defense attorneys going forward about which cases might have the strongest chances of challenging the death penalty itself, said Robert Dunham, executive director of the Death Penalty Information Center, which opposes capital punishment.

"What Justice Breyer does is go through the defects, not to say this is right or this is wrong, but to illustrate how, collectively, they may amount to a system that violates the Constitution," he said. "In that sense, the opinion is not just an invitation for briefing, but a blueprint for the types of arguments that the court would consider in determining whether the death penalty is constitutional or unconstitutional."

Cheryl Pilate, a Kansas City-based attorney who has represented a number of Missouri death row inmates, said it was clear there's a faction on the court that sees fundamental problems with the death penalty that can't be fixed.

"I think Justice Breyer's dissent was a bold and emphatic statement that indicates to us that the issues that have been raised repeatedly are getting close attention, at least among some justices on the Supreme Court," she said. "So we think it bodes well for the long run and in the meantime, there's an awful lot of work to do."

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Associated Press writer Jim Salter in St. Louis contributed to this report.