HUNTSVILLE, Texas (AP) — Lawyers for a black man set for execution Thursday are calling on Texas Gov. Rick Perry to halt the punishment because an expert witness 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 and his office says he has chosen to halt just four executions, including one for a woman who was later put to death.

But Duane Buck’s case is one of six convictions that former 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.

Prosecutors repeated the sentencing hearings in the five other cases — with all five again being sentenced to death — but Buck’s case was never reopened.

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.

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.