In court documents made public Wednesday, prosecutors said the statute has been upheld in other cases, and that judges have ways to protect defendants' rights.
Lawyers for suspect James Holmes filed motions last week asking the judge in the case to declare the law unconstitutional because it would require Holmes' attorneys to give prosecutors potentially incriminating information, such as mental health records, if he pleads not guilty by reason of insanity.
The defense said that violates Holmes' rights, including Fifth Amendment protections against self-incrimination. They said defendants who simply plead not guilty are not required to turn over such information.
Holmes is charged with multiple counts of murder and attempted murder in the July 20 shootings at a suburban Denver theater that killed 12 and injured 70.
Prosecutors have not said whether they will pursue the death penalty. Under state law they have to announce their decision within 60 days once Holmes enters a plea.
That is scheduled to happen Tuesday.
In their motions filed last week, Holmes' lawyers revealed for the first time they were considering entering a plea of not guilty by reason of insanity. But they said they could not give him adequate advice on how to plead unless the judge ruled on whether the insanity law was constitutional.
In their response, prosecutors said the Colorado Supreme Court—which would inevitably be asked to take up the question if the trial judge declared the law unconstitutional—would not act "in this hypothetical and advisory fashion," before Holmes had been acquitted or convicted.
Prosecutors also said the U.S. Supreme Court and the Colorado Supreme Court had already upheld laws requiring defendants to give prosecutors information about their mental health if they plead not guilty by reason of insanity.
They said the judge could issue an order or jury instructions preventing misuse of the information.
Prosecutors noted that the Colorado public defender's office, which is representing Holmes, made similar arguments against the insanity law in two other cases, and both times the trial judges ruled against them.
Follow Dan Elliott at http://twitter.com/DanElliottAP