Harkin said the Arizona senator "has a hard time thinking beyond" his extensive military background, The Des Moines (Iowa) Register reported.
"Everything is looked at from his life experiences, from always having been in the military, and I think that can be pretty dangerous," Harkin said in a conference call with reporters.
McCain graduated from the Naval Academy in 1958 and served in the U.S. Navy as a pilot. He was held as a prisoner of war in Vietnam for five and a half years and is the son and grandson of U.S. Navy admirals.
"It's one thing to have been drafted and served," Harkin said, "but another thing when you come from generations of military people and that's just how you're steeped, how you've learned, how you've grown up."
McCain currently serves as the top Republican on the Senate Armed Services Committee.