I'm not sure that the answer is really a "who' question but more of a location or "where" question. It seems to me that a few key counties (maybe even wards) of a few million votes may have determined this election. Both parties know that certain areas are going to go one way or the other no matter what. Ignore them! Find the few places that you can get a ton of people to vote for your guy and focus there. Build your margin of victory, even if it is slight, there. One point (or vote) more than the other guy wins the game.
To me it seems that the election was won in Philadelphia, Fairfax County VA, Lake county Ohio (or around Cleveland), and in Miami, FL. The rest of the country was punch and counter punch basically equaling each other out.