James Holmes will ask a judge to allow him to change his plea to not guilty by reason of insanity in the 2012 Aurora, Colorado movie theater shooting.
Holmes faces murder and attempted murder charges in the July 20 attack, which killed 12 and injured 70. He will seek the new plea at a hearing today.
No motive has emerged, but Holmes' attorneys have repeatedly said their client is mentally ill. It was reported that he tried to commit suicide in jail.
Prosecutors are seeking the death penalty.
An insanity plea carries risks for both sides. Holmes must submit to a mental evaluation by state doctors, whose findings could be used against him.
The plea also might make it harder for prosecutors to get a conviction, as they must convince jurors that Holmes was sane at the time of the event.