According to John Dean in Worse Than Watergate, in 1965 Barry Goldwater sued people who done something like what the Swift Boat Liars did, and won. Dean suggests that Kerry ought to sue as well, to show that you can't get away with slandering and libeling a candidate. The Liars can easily be shown, I imagine, to have lied recklessly and maliciously, or whatever else it takes to sue for slandering or libeling a public figure.
Will Kerry sue? I suspect not, but it seems to me he has a duty to sue, especially because, as a lawyer, he might not find the process of suing so unfamiliar. Either that or John Edwards could calm him down and tell him it's going to be okay. :)