Here is my take on this. Since 1980, the break has gone to the Republicans (except 84 when there was no break), I believe this is because the Main stream media has made weak kneed voters uncomfortable about publicly supporting the GOP, however when they go to the booth, they will vote with their conscience.
That is a good point, and it ought to be magnified this year, considering both the effects of 9/11 and the incredible pop status that Bush-hating has attained.