The way Benghazi is unraveling it must have occured to the President if he is re-elected, there is a growing possibility of impeachment.
If he is not re-elected there will be no impeachment.
Republicans will hold the house. It takes only a majority to impeach. Repubicans may take the senate. It takes two thirds to convict.
I expect he would survive the trial but it may be, for him, better to lose the election than face the process.
I think he has already made his decision.