The planned guilty plea Tuesday comes after mental health officials concluded Loughner is competent to understand the charges against him in the Jan. 8, 2011, mass shooting that also injured 12 other people at a meet-and-greet political event outside a supermarket in Giffords' hometown of Tucson.
One or more psychiatric experts who examined Loughner, 23, are expected to testify Tuesday in U.S. District Court in Tucson they have concluded despite wide swings in his mental capacity, Loughner understands what happened and acknowledges the gravity of the charges, sources told the Los Angeles Times and The Wall Street Journal.
U.S. District Judge Larry Burns declared Loughner not competent to stand trial in May 2011 and ordered him to be held in federal custody until he could be restored to competency.
In February, a psychologist appointed by the court said Loughner, a diagnosed schizophrenic, remained incompetent to stand trial but was making progress toward competency, Burns said.
The terms of a plea bargain -- in which Loughner agrees to plead guilty to particular charges in return for a concession from the prosecutor -- remained unclear Saturday.
It was possible Loughner would admit guilt to all or just some of the charges in return for a lengthy prison sentence rather than a potential death penalty verdict at trial, the Times said.
Many victims of the shooting and their families are expected to attend the downtown Tucson hearing, not far from the site of the attack during Giffords' "Congress On Your Corner" event.
Giffords, D-Ariz., resigned her congressional seat Jan. 25 to focus on her recovery.