The prevailing wisdom in most presidential elections is that undecided voters tend to break away from the incumbent and towards the challenger. If that is true, then Barack Obama is in trouble. Even if you dismiss the Gallup and Rasmussen polls, he is still polling around 48% nationally. If Mitt Romney gets, say, two thirds of the undecides, that would mean he would get somewhere between 50.5 and 51 percent of the popular vote.
Of course, the exception to that logic was the 2004 election. George Bush was polling at about 48.5 percent with about ten days to go before the election; yet he ended up getting the majority of the undecided vote and defeated John Kerry.
How to reconcile the obvious contradiction of the Bush reelection with the accepted assumption. Simple. The assumption is wrong. So says John Sides, an Associate Professor of Political Science at George Washington University. In a piece he wrote for The Monkey Cage, Sides explains that there is "little evidence" that either side will benefit from a perceived break among this demographic:
I will assume that undecided voters will make a decision that reflects three
things: their party identification, their approval of Obama (as a “referendum”
model would suggest), and how favorably they feel toward Obama and Romney (the
difference between how they feel about each candidate, as a “choice” model would
suggest). I estimate this model on all decided voters and then predict the
choices of the undecided voters based on the model’s results. The model
predicts the choices of decided voters correctly 99% of the time, which is no
surprise given the factors in the model.
The model predicts that these undecided voters will split almost exactly evenly:
50.1% for Obama and 49.9% for Romney. There is substantial uncertainty in this
estimate, naturally. The 95% confidence interval for Obama’s predicted vote
share is 44% to 56%.
Because there is no clear projected winner in national polls at this point,
it is hard to say whether the no-shows would disproportionately hurt Romney or
Obama. Regardless, my analysis, plus this systematic feature of past elections,
suggests that late decisions by the undecided voters may not be the, er,
deciding factor in 2012.
If you're looking for silver linings that buck history and are encouraging, Sides has a very compelling argument to make. Time will tell if he is right.
In the meantime, consider the words of Nate Silver, and I'm paraphrasing here: "You'd rather be Obama up 3 defending than Romney down 3 with the ball."