John Kerry, the Massachusetts Democrat who will fight Mr Bush for the White House said the joke displayed a "stunningly cavalier" attitude.
"If George Bush thinks his deceptive rationale for going to war is a laughing matter, then he's even more out of touch than we thought. Unfortunately for the president, this is not a joke."
He added: "585 American soldiers have been killed in Iraq in the last year, 3,354 have been wounded and there's no end in sight. George Bush sold us on going to war with Iraq based on the threat of weapons of mass destruction. But we still haven't found them, and now he thinks that's funny?"
The statement from Mr Kerry also included a comment from an Iraqi war veteran, Brad Owens.
"War is the single most serious event that a president or government can carry its people into," he said. "This cheapens the sacrifice that American soldiers and their families are dealing with every single day."