William Saletan makes a good point about George W. Bush and suggests what strikes me as a sound criticism to voice of him and his administration. There's an unfortunate tendency to try and cast everything a politician does as a lie: If he takes one position and ten years later changes his mind, someone says he lied about his position. If he believes something that turns out not to be true, someone calls him a liar. To my mind, "liar" implies a conscious effort to deceive and that usually is not the case.
But Saletan argues, and I think he's right, that the Bush administration takes a position and encourages everyone around them to alter the evidence to support it. The whole matter of Stem Cell Research is one of many where almost the entire scientific community is telling Bush he was wrong about his science and he won't admit it. Here's a piece over on Tapped that summarizes this one. The election won't turn on issues like this one but it might matter in this sense: One of the things a lot of people seem to admire about Bush is that they believe he has strong, firm resolve. It might cost him a lot of votes if that resolve is viewed as a stubborn refusal to deal with reality.