WASHINGTON, Sept 4 (Reuters) - Pressing his case against an assault on Baghdad, former U.S. Attorney General Ramsey Clark said on Wednesday the United States had no legitimate reason to attack Iraq and that it would be a grave mistake to do so. "The claim that Iraq is a threat is a complete fraud. I don't think they believe it for a minute," Clark said, referring to the Bush administration's stated grounds for seeking to topple Iraqi President Saddam Hussein. Clark, who served in the Johnson administration at the height of the Vietnam war, said it would be "the...