Holmes' legal team argued in a court filing Friday Colorado's death penalty is sought so infrequently it qualifies as cruel and unusual punishment and the rules governing death penalty trials unfairly bias the jury against defendants.
Citing a study by two University of Colorado law professors Holmes' defense team commissioned in another death penalty case, the attorneys said of the 544 first-degree murder cases in Colorado since 1999, 92 percent met the state's death penalty standard.
Despite that, in only five cases have prosecutors sought the death penalty.
In several motions unveiled Tuesday, Holmes' lawyers argued the state's death penalty statute should be invalidated, The Denver Post reported.
Because so many of the state's first-degree murder cases meet the death penalty threshold, the lawyers argued Colorado's law does not meet the U.S. Supreme Court's stipulation that capital punishment be limited in its use.
And because it has been sought so infrequently -- and only by one district attorney's office, the one prosecuting Holmes -- its application is arbitrary.
Lawyers also argued that jurors seated in a capital murder case are all asked whether they could sentence a person to death, thereby "death qualifying" the jury, and seating a panel that's predisposed to convict the accused.
"Imposition of the death penalty is rare, unusual, freakish and inconsistently applied throughout the State of Colorado," the defense lawyers wrote.
Holmes' lawyers have admitted their client was the gunman in a horrific shooting at a movie theater in Aurora last year, during which he killed 12 people and wounded dozens more at a midnight screening.
Prosecutors are due to respond to the motions this week.