Ed is right; Obama will not have the same turnout this election as he did in the last, just as Mitt Romney's voters will not be identical to John McCain's. At each cycle, the electoral universe changes for a variety of obvious reasons: Some voters die; others become eligible for the first time; some who didn't vote last time decide to turn out; others stay home. Think of the electorate as fans at a Redskins game. There is always a core group who comes to the game, week after week, year after year. But at each game, there is a slightly (or dramatically in the case of preseason games) different group of fans in the stands.
There are a variety of factors that will drive turnout this year and determine the electorate's final composition. Anger at Obama will certainly be a powerful motivator for the Republican base. It also might suppress the Democratic base (although recent polls suggest its enthusiasm is retuning) and cause people who voted for Obama in 2008 to switch. At the same time, the Obama turnout machine is not only contacting previous Obama voters but conducting a systematic search for new ones. Moreover, and most promisingly for Obama, this election is being framed, so far, not as a referendum on Obama, but as a choice.