Death row inmate Donnie Edward Johnson is scheduled to be executed on May 16. But with days to go, he still hasn't decided how he wants the state to kill him.

Because Johnson, 68, was convicted of a murder that took place before 1999, he can choose between two methods: lethal injection, Tennessee's primary method, or the electric chair, the back-up option.

Eligible inmates typically make their choice a month before their execution date. If they don't choose, the state plans for a lethal injection.

Johnson is on death row for the 1984 murder of his wife Connie Johnson. He has filed a clemency application asking Republican Gov. Bill Lee to grant him mercy and stop the execution.

Prison officials will continue preparations for the execution until a decision is announced. Johnson's lawyers say he has intentionally withheld his execution method decision — for now.

That is because the U.S. Supreme Court is considering a challenge to Tennessee's lethal injection drugs. Johnson doesn't plan on making a choice until the nation's high court has spoken.

Supreme Court to weigh in on lethal injection challenge

The challenge, brought by Johnson and 22 other death row inmates, argues that the state's three-drug protocol does not keep inmates from feeling excruciating pain as they die. The pain is so severe, they say, that it is unconstitutional.

Tennessee courts rejected the inmates' lethal injection challenge in 2018, but the inmates are asking the Supreme Court to revive the challenge based on state law that they said hamstrung their constitutional right to challenge "cruel and unusual punishment."

The justices were slated to debate the case Thursday. Johnson's attorneys expect a ruling Monday, three days before the scheduled execution. Federal public defender Kelley Henry said they would discuss a choice with Johnson then.

A wave of executions began in Tennessee last year, including one lethal injection. But the same argument has derailed executions in Ohio.

Ohio Gov. Mike DeWine, a Republican, delayed lethal injections earlier this year after a federal judge fiercely criticized the trio of lethal injection drugs used in Ohio, which are the same ones that are used in Tennessee.

Experts say the first of the three drugs, the sedative midazolam, does not dull the pain caused by the following two.

Inmates say Tennessee law violates constitutional rights

Tennessee courts said that, regardless of the pain, the inmates had failed to prove other execution drugs were readily available.

In their challenge to the U.S. Supreme Court, the inmates' lawyers argue Tennessee law shielding details about lethal injection drug procurement made it impossible to say what alternative drugs might be available to the state.

The inmates argued the law doomed their efforts and violated their due process.

"Tennessee law ensured that petitioners’ claim would fail," the inmates attorneys argued in their petition to the high court. "Tennessee’s execution secrecy statute barred discovery into the state’s communications with 10 concededly willing suppliers. It also barred petitioners from exploring the details of the suppliers’ offers by deposing those suppliers or even the state officials with whom they interacted."

State lawyers are opposing the effort to revive the lethal injection challenge, saying the inmates never raised this issue before and that other rulings have maintained the constitutionality of state shield laws for executions.

Last year, two death row inmates in Tennessee chose the electric chair over lethal injection amid fears that the injection led to several minutes of torturous pain before death.

Reach Adam Tamburin at 615-726-5986 and atamburin@tennessean.com. Follow him on Twitter @tamburintweets.