“The decision was made to delay the resignation apparently to avoid potential embarrassment to the president before the election,” an FBI source told me. “To leave him in such a sensitive position where he was vulnerable to potential blackmail for months compromised our security and is inexcusable.”
November 12, 2012
"FBI agents investigating CIA Director David Petraeus's affair were shocked when told by bureau officials..."
"... that despite the national security implications, no action would be taken on their findings until after the presidential election: Only then would President Obama ask for Petraeus’ resignation."