Clarence Brandley was on death row in Texas until he won such an appeal.

"It has been proven and shown over and over again that Texas has executed innocent people," he said.

Brandley says many poor black men are sent to the death row facility near Livingston, Texas, because they lacked the money to mount an effective defense.

Iranian immigrant Hooma Hedayati, who is with a group called "Witness to Innocence," organized a tour of Texas cities by exonerated prisoners.

"Several studies have shown that innocence is one of the most important factors in people deciding whether they are for or against the death penalty and what we are doing is giving a face to innocence, where people get firsthand to experience people who have been wrongfully convicted," Hedayati explained.

Another argument against the death penalty is that it is ineffective, says Dorothy McClellan, who teaches at Texas A & M University at Corpus Christi.

"It does not accomplish what it sets out to do. It seeks to get revenge, but it is inefficient, it does not, in fact, deter," she said.

But many Americans believe the death penalty dissuades people from committing grisly murders, and, at the very least, guarantees they will not kill again.

Opponents try to counter that notion by showing that, in some cases, the person being executed is not the one who committed the crime.