HUNTSVILLE, Texas (AP) -- Attorneys for a man scheduled to be put to death in Texas on Thursday are asking Gov. Rick Perry to halt the execution amid questions about the role race played in the sentencing.

Duane Buck's case is one of six convictions that then-Texas Attorney General John Cornyn - a political ally of Perry who is now a Republican U.S. senator - reviewed in 2000 and said needed to be reopened because of the racially charged statements made during the sentencing phase of the trial. A psychologist told jurors that black criminals were more likely to pose a future danger to the public if they are released.

Perry, the frontrunner for the Republican presidential nomination, is an ardent supporter of capital punishment. During his 11 years in office, 235 convicted killers have been put to death in Texas. His office says he has chosen to halt just four executions, including one for a woman who was later put to death.

If courts continue to reject Buck's appeals, only Perry could delay the lethal injection by invoking his authority to issue a one-time 30-day reprieve for further review. Perry's actions are being closely watched, particularly by death penalty opponents, after he said during a presidential debate that he has never been troubled by any of the executions he's overseen as governor.

In the five other cases Cornyn said needed to be reopened, prosecutors repeated the sentencing hearings and the defendants were again sentenced to death. Prosecutors contend Buck's case was different from those and that the racial reference was a small part of a larger testimony about the prison population.

Buck, 48, was convicted of gunning down ex-girlfriend Debra Gardner, 32, and Kenneth Butler, 33, outside Houston in July, 30, 1995, a week after Buck and Gardner broke up. Buck's guilt is not being questioned, but his lawyers say the jury was unfairly influenced and that he should receive a new sentencing hearing.

A third person, Buck's stepsister, Phyllis Taylor, also was wounded, though she has since forgiven Buck and sought for his death sentence to be commuted to life in prison.

Gardner's 14-year-old daughter and 11-year-old son were among those who witnessed the shootings. Officers testified that Buck was laughing during and after his arrest, saying Gardner deserved what she got.

The Texas Board of Pardons and Paroles, all of whom are Perry appointees, denied Buck's clemency request Wednesday, and the 5th U.S. Circuit Court of Appeals subsequently rejected his appeal.

Buck's lawyers contend the case was "tainted by considerations of race" after psychologist Walter Quijano testified in response to a question from lead prosecutor Joan Huffman that black criminals are more likely to be violent again in the future. Whether or not someone could be a continuing threat to society is one of three questions Texas jurors must consider when deciding on a death sentence.

Cornyn said in a news release in 2000 that a half-dozen capital case sentences, including Buck's, needed review because of Quijano's testimony at their trials.

A spokesman for Cornyn declined to comment.

Lucy Nashed, a spokeswoman for Perry, said that because the governor will be out of state, Lt. Gov. David Dewhurst will preside over the execution. She declined to comment further about Buck's case.

Any final order to delay would technically come from Dewhurst. However, Perry's office frequently points out that Perry remains the governor and in contact with Austin while traveling. Dewhurst spokesman Mike Walz said the lieutenant governor does not comment on pending executions.

Huffman, now a state senator and one of Perry's closest allies in the Legislature, defended asking Quijano the racially charged question, saying, "I have absolutely no concern whatsoever." She noted that Quijano was a defense witness, her question came in reference to a report he prepared for the defense and the issue was raised just once.

However, Huffman's assistant prosecutor in the case, Linda Geffin, has joined the call for a new sentencing hearing, saying Wednesday that "race should never be put in front of a jury in any case, particularly a death penalty case."

The execution would be the second this week and the 11th this year in Texas. Two more Texas prisoners are set to die next week.

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Weissert reported from Austin.

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