A member of the Senate Armed Services Committee, McCain said increasing U.S. forces would require expanding the standing Army and Marine Corps - a step the Bush administration has resisted. He also reiterated his opposition to a hasty U.S. withdrawal.
"If we leave ... the fighting will evolve into chaos there," McCain told reporters after speaking at an event for local Republican candidates.
Reporters asked him to elaborate on his statement last week in Iowa that more combat troops are needed in Iraq to quell a "classic insurgency."
"Another 20,000 troops in Iraq, but that means expanding the Army and the Marine Corps," he said.
"It's not just a set number."
Blow into that balloon Senator McCain!