If Bushco decide to pull an election day terror alert, it won't hurt us at all, and could help. Those who are suspicious of Bush will assume he is lying and make sure they go to the polls to throw his ass out. Those who think he is telling the truth will be cowering in their homes, afraid to go out.
So I don't think issuing a warning can hurt us anymore.
But if there is a real attack, it's a different story. There are several possible scenarios, with different outcomes:
We are attacked without warning. Bush appears incompetent - he can't protect us.
We are attacked after having been warned, but the attack doesn't get prevented. It's hard to say how people will react.
We are attacked after having been warned, and the attack is largely foiled. Bush appears competent and people will rally around him.