WASHINGTON -- Trading sharp words, a deeply divided Supreme Court upheld the use of a controversial drug in lethal-injection executions yesterday, even as two dissenting justices said for the first time they think it's "highly likely" the death penalty itself is unconstitutional.

On their last day together until the fall, the justices voted 5-4 in a case from Oklahoma that the sedative midazolam can be used in executions without violating the Eighth Amendment prohibition on cruel and unusual punishment.

The court also divided 5-4 in cases upholding congressional districts drawn by independent commissions and calling into question first-ever limits on mercury emissions from power plants. The justices also agreed to hear an important affirmative action case in the fall.

In the dispute over the lethal-injection drug, midazolam was used in Arizona, Ohio and Oklahoma executions in 2014. The executions took longer than usual and raised concerns that the drug did not perform its intended task of putting inmates into a coma-like sleep.

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

In a biting dissent, Justice Sonia Sotomayor said, "Under the court's new rule, it would not matter whether the state intended to use midazolam, or instead to have petitioners drawn and quartered, slowly tortured to death, or actually burned at the stake."

Alito responded, saying "the dissent's resort to this outlandish rhetoric reveals the weakness of its legal arguments."

In a separate dissent, Justice Stephen Breyer said the time has come for the court to debate whether the death penalty itself is constitutional. Justice Ruth Bader Ginsburg joined Breyer's opinion.

"I believe it highly likely that the death penalty violates the Eighth Amendment," Breyer said, drawing on cases he has reviewed in more than 20 years on the Supreme Court bench.

More than 100 death row-inmates have been exonerated, showing that the death penalty is unreliable, Breyer said. He said it also is imposed arbitrarily, takes far too long to carry out and has been abandoned by most of the country. Last year, just seven states carried out executions, he said.

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The two senior liberal justices joined retired Justice John Paul Stevens, who was in the courtroom yesterday, and Justice Harry Blackmun in late-career pronouncements calling into question the use of the death penalty, although Breyer and Ginsburg stopped short of declaring their outright opposition to capital punishment.