From a recent article that I was reading in preparation for the debates:
Obviously, Bush doesn't sound this way as President, and there is no one conclusive explanation for the change. I have read and listened to speculations that there must be some organic basis for the President's peculiar mode of speech: a learning disability, a reading problem, dyslexia or some other disorder that makes him so uncomfortable when speaking off the cuff. The main problem with these theories is that through his forties Bush was perfectly articulate. George Lakoff tried to convince me that the change was intentional. As a way of showing deep-down NASCAR-type manliness, according to Lakoff, Bush has deliberately made himself sound as clipped and tough as John Wayne. Moreover, in Lakoff's view, the authenticity of this stance depends on Bush's consistency in presenting it. So even if he is still capable of speaking with easy eloquence, he can't afford to let the mask slip.
I say: Maybe. Clearly Bush has been content to let his opponents, including the press, think him a numbskull. Even his unfortunate puzzled-chimp expression when trying to answer questions may be useful: his friends don't mind, and his enemies continue to underestimate him. But to me the more plausible overall explanation is the sheer change in scale from being governor of Texas to being President of the United States.
I had this exact conversation with Sandcastle several months ago.
His response was that Karl rove has never told anyone to act stupid in order to get elected. Well I just had my idea validated in a major publication. I like being me sometimes.