My guess would be much closer to election time so that the administration does not have time to recover. If one presumes that the goal is not only to terrorize, but also to affect the outcome of the election, I would expect a multitude of small attacks in predominantly strong republican areas designed to keep people away from the polls on election day. I would also suspect a communication instructing America to bow to UN authority or something more serious would follow.
Just my 2 cents.
Major power grid breakdown during the election - electronic voting screens would go down, votes can't be counted.