There’s a nice summary by Charles Franklin at Pollster.com (see here) of the National Election Studies analyses dating back to 1948 regarding how the undecideds break in presidential elections. As I noted in an earlier post, the basic rule of thumb is that challengers do slightly better than incumbents, but that doesn’t apply in this election since there’s no incumbent (although Obama would have you believe that McBush is the current officeholder!) Franklin notes that there’s rarely a lopsided split toward one candidate or the other. The most recent one-sided split occurred in 2000, when about 7 in 10 (66%-23%) of the undecideds broke for Bush over Gore. But usually the break is closer to 55-45, which is how Pew allocated their votes for McCain and Obama in the poll I cited yesterday, and which is consistent with how leaners break when pushed by pollsters.
The other point Franklin makes is one that Jesse Gubb brought up yesterday: a substantial number of undecideds just don’t vote. Based on the graph, a rough “guestimate” is that 10-15% of undecideds don’t bother voting. Of course, given the small subsample sizes, there’s a lot of uncertainty in these estimates. And Franklin doesn’t bother telling how undecideds are defined either – is it voters who have not made up their mind in the last day? The last week?
What does this suggest for 2008? As Bert and I have both noted, there’s no clear pattern across elections that seems to explain how undecideds break, except perhaps for a slight partisan effect that Bert picked up in his analysis. Based on the demographics of the undecideds, and the results in those races that have tightened in recent days (e.g., in Pennsylvania where McCain is winning 5-1 among late deciders), I continue to believe that the majority of the undecideds will go to McCain, but not in great enough proportions to win the election for him. The exit polls will give us an early indication regarding how those who come to a decision in the last two weeks voted. Interestingly, if you look at the tracking polls so far (and I’ll recheck this data today) the number of undecideds is holding steady at about 6%. That’s why Pew, in their final poll, had to make a decision to allocate them.
I just want to be clear here: given current polling figures, even if the undecideds break substantially for McCain, it won’t be enough to give him a victory in the popular vote. It may be enough to put him over the top in key battleground states, however. I’m doing a longer analysis of that now.