Cause and effect reversed?
You could, of course, turn the numbers around and make an equally valid case that a rise in terror threats would be more likely anyway as you get closer to an election, as in Spain. I notice the latest threats from bin Laden and "Azzam the American" seemed pretty closely timed.
And we do have at least one data point--September 11--which shows that the threat is not entirely imaginary.
And we don't have to be that cynical--maybe the FBI has been doing a good job.