MSNBC has a nice graphic up that shows terrorist attacks related to al Qaida from the late '80's to the present. (Scroll down to the "Time Line" graphic.)
Prior to Bush taking office, there were 22 successful or attempted attacks. More than 50%of these (12 of the 22) were thwarted. There were no attacks during at least one full year of Clinton's presidency.
Since Bush took office, there have been 22 successful or attempted attacks. Of these, only 4 were thwarted. So that's, what, a hair over 20%? (Math was always my weakest subject).
The numbers are not on Bush's side.