In The Blank Slate, Steven Pinker argues that “the best liar is the one who believes his own lies”.
I wonder if this has anything to do with who succeeds in politics. Often, people think favorably of those who promise things they want. At the same time, they want people who seem genuine. The ideal candidate, then, is someone who genuinely believes that they will keep their promises. That is easier to do when your plans are vague or when you assume the sheer force of your personality will produce your desired outcome.