Slate's William Saletan has an excellent piece that the Democrat presidential hopefuls should heed.
I'm not excusing the games Republicans play. But by projecting all evil onto Republicans, Democrats spread the same political disease: the notion that you don't have to be wary of lying or cheating unless the other side is doing it. Lying and cheating don't belong to Republicans or Democrats. We're all susceptible, and we're all guilty.
That may be heretical to some on the loony left (and on the far right too), but it's true.