Hmmmmm how ever could you influence who would win an election? Hmmmmmmm, Let's see. Oh yeah, by voting.
May I remind you that Obama was not running against an incumbent? (Not to mention that he mostly ran on hope. And after the war mongering Bush it really didn't take much.) When running against an incumbent, you have to go to the centre. That was how Clinton and Biden won. And we're back to how do you move things left? Simple: By voting. By giving consistent and overwhelming victories.