Volokh explores the legality of forging documents for political purposes and finds few laws on point (clearly forging documents for commercial gain would break all kinds of laws – as would forging documents as part of a recognized tort, such as slander).  As occurred to me during the Swift Boat Veteran debacle, we are better protected against lies about toothpaste and cat litter, then we are against lies about (and by) our government (and about reasons for going to war for that matter).