DENVER, July 22 (Reuters) - The lead prosecutor in the Colorado movie massacre trial tore into the state's governor at a news conference, calling him arrogant and weak for giving the mass murderer a reprieve from execution.

This was two years ago, and Arapahoe County District Attorney George Brauchler was not talking about James Holmes, who last week was found guilty on all counts by a jury for fatally shooting 12 people and wounding 70 at a midnight premiere of a Batman film in July 2012.

At the time, Brauchler was responding to Governor John Hickenlooper's decision to grant a temporary reprieve in an earlier Denver-area mass killing. That case sheds light on the political sensitivities surrounding the ultimate punishment in Colorado.

On Wednesday, the jury which convicted Holmes on 165 counts of first-degree murder, attempted murder, and explosives charges begin the penalty phase of the trial. After hearing more weeks of testimony, they will decide if the California native is to be executed by lethal injection, or serve life in prison with no possibility of parole.

Colorado has executed just one inmate in nearly 50 years. Still, a Denver Post poll last year showed 63 percent of state residents surveyed support the death penalty. In the case of Holmes, a separate poll by the newspaper this week showed an overwhelming 70 percent favored execution for the former neuroscience graduate student. That poll had received more than 5,800 votes by Tuesday afternoon.

Two years ago, Brauchler called his news conference at the state Capitol to denounce Hickenlooper's granting of a so-called "temporary reprieve" to the state's longest-serving death row inmate. Nathan Dunlap was convicted in 1996 of killing four workers at a pizza restaurant where he had recently been fired.

The temporary reprieve meant the governor's successor could reinstate Dunlap's death sentence, and the prosecutor decried the decision as indecisive, and "clemency light."

"You hear frustration and anger in my voice because those victims that have waited patiently for justice for 20 years will now wait for years more," Brauchler told reporters at the time.