Chuck,
The thinking by the pollsters is that Bush has not only high disapproval ratings, but that there are a huge number of "undecided" voters. While it is still early, whenever a voter is generally undecided it more often than not turns into a vote for the challenger. The reasoning is that everyone knows Bush because of his role and there are no real secrets you will learn about him between now and November. If he hasn't sold you yet in 3.5 years of office that he deserves another term, chances aren't good that he will convince someone in the next 6 months. It is all theoretical, but I have seen a number of independent pollsters and pundits mention this fact and compare how Presidents with this level of approval have all failed in re-election in the past.