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As Ohio prepares for its first execution in more than three years, death-row inmates are asking the U.S. Supreme Court to postpone lethal injections once again.

The state has not put anyone to death since the 2014 execution of Dennis McGuire, who appeared to gasp for breath during the 25 minutes it took for an untried cocktail of drugs to end his life.

A federal appeals panel last month overturned a lower court ruling blocking the state from proceeding with executions using the controversial sedative midazolam because of the risk of a painful death.

Ronald Phillips' execution is scheduled for July 26. Uncredited / AP

That decision clears the way for Ohio to execute Ronald Phillips on July 26, unless the high court steps in. Lawyers for Phillips and two other death row inmates asked the justices Tuesday to issue stays of execution.

"The judges who have heard the witnesses have found their concerns about the severe pain that a midazolam-based protocol will cause to be compelling," the petition says.

"There is mounting evidence that the three-drug protocol Ohio seeks to use is unconstitutionally painful."

Phillips, who was convicted of raping and murdering his girlfriend's three-year-old daughter in 1993, has already won several previous reprieves, including a delay as officials considered, and ultimately rejected, his request to donate organs to his family members.