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.