Giffords, 40, suffered a single gunshot wound through the brain. Although initial news reports indicated she had died, the congresswoman survived and underwent surgery at the University Medical Center in Tucson.
"I can tell you at this time, I am very optimistic about her recovery," Dr. Peter Rhee, medical director of the hospital's trauma and critical care unit, said at a news conference.
"We cannot tell what kind of recovery, but I'm as optimistic as it can get in this kind of situation," adding that the next 24 hours will be key in determining her prognosis.
Among the six confirmed dead were Chief Judge John Roll, of the U.S. District Court for Arizona, and a 9-year-old girl. At least 12 additional people were wounded in the attack.
Law enforcement officials have identified the gunman as Jared Lee Loughner, 22. The suspect is currently in custody.