Karl Rove --
A candidate can improve his party's performance by getting additional people out to vote and persuading people inclined to support the other party to cross over. The first yields an additional vote; the second is worth two, the one a candidate gets and the one he takes away from his opponent.
He ignores the tactic of preventing the opponent's supporters from voting (or for that matter, having their votes counted). Rove correctly identifies new voter mobilization as a key part of Obama's voting bloc. It helps understand why there was so much yelling from the right about Acorn, since in a Rovian stroke of genius, Obama was taking the other side's strongest tactic -- voter suppression -- and working against it.