By Jon Herskovitz

AUSTIN, Texas (Reuters) - Texas is set to put to death convicted rapist and murderer Robert James Campbell on Tuesday in the first execution in the United States since a botched lethal injection in Oklahoma in April raised questions about capital punishment in the country.

Lawyers for Campbell are seeking a stay, saying the problems in Oklahoma and secrecy surrounding the providers of execution drugs demand a halt to allow for a sober reflection on how the death penalty is carried out.

Texas plans to execute Campbell, 41, at 6 p.m. CDT (2300 GMT) by a lethal injection of the sedative pentobarbital at its death chamber in Huntsville. He would be the 21st inmate executed this year in the United States.

"The Texas Department of Criminal Justice does not have plans to change its execution protocol based upon the Oklahoma incident," the department said in a statement.

Oklahoma halted the April 29 execution of convicted murderer Clayton Lockett while it was in progress after what prison officials said was a blown vein that made them unsure if the lethal cocktail was being properly administered.

Lockett, mumbling and in apparent pain on a prison gurney with an IV in his groin, died of an apparent heart attack 43 minutes after the procedure started. The execution was the state's first use of a new three-drug lethal injection mix.

"We are in a new era since just two weeks ago. The public has not been closely engaged in the lethal injection debate until that case broke in Oklahoma," said Richard Dieter, executive director of the Death Penalty Information Center, which monitors capital punishment.

The White House said the Oklahoma execution failed to meet humane standards. President Barack Obama said it raised questions about the death penalty and that he would ask the U.S. attorney general to look into the situation.

Campbell was convicted along with a co-defendant of kidnapping Alexandra Rendon from a Houston gas station in 1991. The pair drove her to a desolate area, raped her and then told her to run.

Campbell then shot her in the back and left her to die, stealing her car to get away.

"Words can't describe the brutality of this crime," Israel Santana, a cousin of Rendon and a Houston defense attorney, told Reuters. "I can't imagine what her final few hours were like. This guy deserves what he has coming to him."

Oklahoma, Texas and other states have been scrambling to find new suppliers and chemical combinations after drug makers, mostly in Europe, imposed sales bans because they objected to having medications made for other purposes being used in lethal injections.

The states have turned to lightly regulated compounding pharmacies, which can mix chemical for medicines, to supply the drugs, while trying to keep the name of supplier secret.

Lawyers for death row inmates have argued there may be problems with purity and potency of the chemicals that come from these compounding pharmacies, raising questions about whether they should be used to prepare lethal injection drugs.

An attorney for Campbell said it was time for states to provide information about who makes and supplies the drugs.

"This is a crucial moment when Texas must recognize that death row prisoners can no longer presume safety unless full disclosure is compelled so that the courts can fully review the lethal injection drugs to be used and ensure that they are safe and legal," said Maurie Levin, a lawyer for Campbell.

Texas has executed 515 prisoners, more than any other state, since the Supreme Court reinstated the death penalty in 1976.

(Reporting by Jon Herskovitz; Editing by Edith Honan and Mohammad Zargham)