Newsweek says foreign entities compromised computers of both McCain and Obama:

Officials at the FBI and the White House told the Obama campaign that they believed a foreign entity or organization sought to gather information on the evolution of both camps’ policy positions—information that might be useful in negotiations with a future administration. The Feds assured the Obama team that it had not been hacked by its political opponents. (Obama technical experts later speculated that the hackers were Russian or Chinese.) A security firm retained by the Obama campaign took steps to secure its computer system and end the intrusion.

Maybe they were after position papers, or maybe they were trying to get data on who would win so they could place bets with more certainty. If nothing else, it brings to light that physical security of the candidates was top notch whereas cyber security apparently was more of an afterthought.