The better question is why did the Republicans gain in the house but lose the Senate and Presidency. Obviously any fixing was targeted. They have proved the past 4 years they only need the Senate and Presidency. They have already compromised the Supreme Court. Ask yourself why Indiana made such a swing against Obama, but Ohio, Wisconsin and Florida did not ?
Florida is the state I would suspect fraud the most. Everything pointed to Romney winning there. Most reliable state polls gave him a safe lead.