Interestingly enough, the number of non voters in 2012 was higher than the number of votes for either Obama or Romney.
So if all the people that didn't vote (many because they feel that their vote doesn't change things anyway) voted for a third party candidate, that candidate would win without having to take a single vote from either of the two main party candidates.
Third parties don't win, but they do affect outcomes. If Perot hadn't gotten back in in 1992, Bush would probably have been re-elected. No Clinton presidency, no impeachment, no Hillary. Would be quite a different political landscape by now.
That candidate would win the popular vote, but that doesn't mean they'd win the electoral vote. All people who's votes counted voted in the last election...
So if all the people that didn't vote (many because they feel that their vote doesn't change things anyway) voted for a third party candidate, that candidate would win without having to take a single vote from either of the two main party candidates.