Mitt Romney is a frequent and blatant liar. Why don't our guardians of discourse care?

The central point is this: Mitt Romney is running a staggeringly dishonest campaign, one that traffics repeatedly in known lies. Why is that, itself, not more of an issue? Our political standards have certainly gone down the crapper in recent decades, but centering an entire campaign around falsehoods seems like one of those things that all the sensible political villagers would, in theory, get more than a bit miffed about.