They try dumb whenever they can. (This is why the one attack that unhinges liberals is to call them stupid. It's the one thing they can't think of themselves as being. As a mathematician, I can easily get away with it.)
But Nixon was not stereotyped as dumb. He was always portrayed as evil, manipulative, secretive, anti-social, etc., but dumb never took hold.
Remember GHWB? He was a "wimp". Even when he attacked Iraq, that was portrayed as some sort of attempt to quash the "wimp factor".
Poor Palestinian Politics Injures Hillary and Our Righteous Cause