HUNTSVILLE, Texas — Attorneys for a black man set to die this evening for a double slaying in Houston 16 years ago appealed to the U.S. Supreme Court and Texas Gov. Rick Perry to block the execution because a question about race was asked during the punishment phase of his capital murder trial.

Duane Buck, 48, faced lethal injection for fatally shooting his ex-girlfriend and a man in her apartment. In an appeal to the Supreme Court today and a letter to Perry, Buck’s lawyers said a psychologist testified that black people were more likely to commit violence.

“The State of Texas should not condone any form of racial discrimination in the courtroom,” attorney Katherine Black wrote Perry, urging the governor to use his authority to issue a one-time 30-day reprieve for Buck. “The use of race in sentencing poisons the legal process, undermines the reliability and fairness of the sentence, and breeds cynicism in the community toward the very institution entrusted with protecting the rights of all persons equally.”

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 racial reference.

In the other five cases, new punishment hearings were held and each convict again was sentenced to die. State attorneys contend Buck’s case was different from the others and that the racial reference was a small part of larger testimony about prison populations.

Perry is a capital punishment supporter and as frontrunner for the Republican presidential nomination his actions now are coming under closer scrutiny. During his 11 years in office, 235 convicted killers in Texas have been put to death. His office said he has chosen to halt just four executions, including one for a woman who later was executed.

Buck, 48, was convicted of gunning down ex-girlfriend Debra Gardner, 32, and Kenneth Butler, 33, outside Houston on 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 psychologist, Walter Quijano, was a defense witness and the testimony at issue came in response to a question from a prosecutor. Jurors in Texas must decide on the future danger of an offender when they are considering a death sentence.

Buck’s attorneys went to the Supreme Court after losing appeals in lower courts. A clemency request to the Texas Board of Pardons and Paroles, all of whom are Perry appointees, also failed.

Assistant Attorney General Edward Marshall told the Supreme Court Buck’s appeals were attempts to relitigate claims that every court, including the Supreme Court, already rejected.

“The record in Buck’s case reveals that no constitutional violation occurred during his sentencing trial,” he told the justices.

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 was not in the state today, meaning any final order to delay would technically come from Lt. Gov. David Dewhurst. However, Perry’s office frequently points out that he remains the governor and in contact with Austin while traveling.

Mike Walz, communications director for Dewhurst, said today he would not comment “while any legal actions are still pending.” Perry generally has adhered to the same policy.

The lead prosecutor who tried the case, Joan Huffman, now a Republican state senator, said this week she had no concern about asking Quijano the question about race. She noted her question came in reference to a report he prepared for the defense and the issue was raised just once.

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.