I think this is where PolitiFact overreaches. They aren’t checking — reasonably, couldn’t check — every single thing candidates say or even every significant fact they cite. So expressing truth-telling as a percentage doesn’t work. It’s not a random sample — it’s the collection of facts they’ve decided to check out, which is always going to skew negative. (Just like Snopes, which is far less politically charged, isn’t going to have a whole bunch of “Oh yes, that’s true.”)

PolitiFact | The PolitiFact report card on the presidential candidates.