Wouldn't It Be Nice If p, Therefore, p (for a moral p)

Instances of the argument-schema ‘Wouldn't it be nice if p, therefore, p’ are usually fallacious, but for a moral p they are not clearly as bad as they are elsewhere. I offer a diagnosis of this phenomenon, arguing that in some conditions – specified in terms of a logical property of the relevant normative operator – such arguments are actually valid (for a moral p).