First, a defamation suit would require proving that Murtha's comments about Haditha were false, and the latest reports don't look good for our side:
Evidence collected on the deaths of 24 Iraqis in Haditha supports accusations that U.S. Marines deliberately shot the civilians, including unarmed women and children, a Pentagon official said Wednesday.Also troublesome for this suit is the claim by the Marine's lawyers that Murtha made his comments about Haditha outside the scope of his role as a congressman. (This claim is likely being made because Murtha, like any congressman, has immunity for comments made pursuant to his role as a member of Congress.) The law on this seems a bit sketchy, you can read way more about that here.
I'd like to know more about who this lawyer is and who's paying for this lawsuit.