Another way to look at it is that 18% of the people who voted for him are unhappy because he's not sufficiently Leftist. Just because some people are unhappy with him does not mean that they will vote Republican in 2012.
Yours is a view taken and espoused by many on the Left to explain Obama’s tanking (real) poll numbers. They say he is not liberal enough.
Sorry, ain’t a buyin’ it.