A conman's modus operandi is to fleece a victim, and then move on to the next one. He can't keep fleecing the same victim, because that victim is wise to him. And because that victim will inform on him, the con man must move to the next town, or even next state, where he can ply scams anew.
This is why the Presidency is not a good job for a con man. Once the public has caught on to your dishonesty, you can't exactly move to the next state. After your actions have belied your words a certain number of times, people are less inclined to believe you, no matter how smooth a talker you are. Even if you half-believe your words yourself.
Barack Obama, after having said he believed in the public financing option, after having campaigned as the post-partisan and post-racial President, after having said that there would be no lobbyists in his administration, after having said that there would be no earmarks in any bill he supported, after having said that all negotiations on his health care bill would be transparent and shown on C-Span, after having said that he would not support a health care bill which would raise taxes one dime, and after having stated that he intended to choose a Supreme Court nominee who was a strict Constitutionalist, has recently cast himself as a deficit hawk.
Sadly, the type of people who do best at politics (and often, business) are by nature con men. But the narcissism inherent in being good at self-promotion is pretty much a guarantee that you won't be a good leader. Because real leadership requires self-awareness, integrity, and the ability to see both sides of an argument. These qualities tend not to coexist with glibness, self-righteousness, and a strong sense of entitlement.
What would be ideal is a system whereby a group of wise men pluck a somewhat unwilling man from obscurity to be President. As the story goes, George Washington had to be talked out of his reluctance (if not out of obscurity).
I'm not seriously suggesting such a system (who would pick the wise men?) But if it were workable, it would certainly beat our current may-the-best-conman-win system.