In addition to the obvious terrible devastation additional attacks would produce, this would be a "damned if you do" type of scenario for Bush.
If the elections were postponed due to such a circumstance, the libs would cry foul and claim it's a political move designed to give the sitting President more time to rise in the polls.
If the elections were allowed to go on despite a major attack, the same libs would cry foul again. Why? Because any "major" attack would likely be in large metropolitan areas. i.e. liberal democrat strong holds.
These libs would then call for another voting date (or at least ten dozen recounts) until they get their desired results.