When people in my field or the broader political discourse argue over undecided voters it is rarely productive because the subject is mischaracterized. We lionize the Undecided Voter. Culturally, we believe that they are better people than the rest of us, for they are doing the idealized version of our Civic Duty: giving both candidates a fair hearing, earnestly debating the merits of each, and not deciding based on partisanship. We believe undecided voters are undecided because they have absorbed both candidates' messages and honestly can't indicate a preference.
Cute story. Too bad that the vast majority of undecided voters are actually "the biggest idiots on the planet" as Brian Griffin said. Sure, the Mayberry image accurately describes some. There are people out there with mixed ideological beliefs who really can't decide. But the remainder, the people who essentially determine every election that isn't a blowout, are undecided because they haven't paid the slightest bit of attention to the race(s).
Let me regale you with an anecdote. In 2004 I was asked by the local paper to sit with a group of "undecided voters" as they watched the first Bush-Kerry debate. At their request I said a few words beforehand, then I fielded my first question:
"Who's the guy running against Bush?"
Followed by, from a different person:
"Which guy is which party?"
It went downhill from there. Anecdotal evidence, but it left a powerful impression on me. Watch any of these "interactive" let's-see-what-the-normal-folk-think circle jerks that the media love so much during the conventions, debates, and so on. Excepting the minority of truly undecided voters, these people simply have no idea what in the hell they're talking about.
Think of the electorate this way. Both parties start out with a baseline of support (40% GOP, 40% Democrat). No major party candidate has failed to win more than 38% of the vote in a two-way race. So Obama and McCain start at 40%. Each gets another 5% of "leaners" who haven't carved a decision in stone but essentially know what they're doing. Then, squared off 45% to 45%, the campaigns battle for the remaining 10% of likely voters.
Here's the rub: most of that determinative group are essentially deciding at random. Their opinions differ based on whatever happened most recently (a debate, announcing running mates, conventions, etc), hence the day-to-day variance in polling. Barring a major gaffe, everything that will move public opinion in the next 5 weeks will be forgotten by election day. Alan Abramowitz popularized (and provided data to support) this "minimal effects" theory of campaigns in the mid-80s. He argued that looking at economic indicators and incumbent presidential approval ratings in May has more predictive power than pre-election polling. The effects of any campaign event are forgotten almost immediately. Do you remember anything from the first Obama-McCain debate? It was, by design, entirely unmemorable. The goal is simply to avoid shooting oneself in the foot; the potential benefits are negligible, so the candidates play defense. But if you're an "undecided" (i.e., clueless) it is the only information on which to base a preference. Until the next one comes along in a few days.
In short, because undecided voters move whichever way the wind is blowing, polling creates a misleading impression of campaign effects. Think of it as a flat line with numerous peaks and valleys, each followed by a return to the mean as the causal events fade from memory. Now here's the rub.
There is absolutely no logic to how these uninformed/undecided voters will make up their minds (if they bother to vote). Lacking substantive knowledge, they'll choose at the last minute based on whatever fragmentary piece of information they can recall on the way to the voting booth. Or they'll pick the candidate they find more attractive (seriously). Or they'll pick the one whose spouse seems nicer. Or they'll pick the one with the best hair. Or maybe they'll flip a coin. This is how the 5-to-10 percent of the electorate that will determine the outcome of the race makes up their minds. This is why the candidates are so god-awfully repetitive with their thematic talking points ("Hope" or "Change" or "America First" or "Bunnies"). They know that 80-90% of us have paid attention and made up our minds. Some of the remaining 10-20% are leaning one way or legitimately unsure (but informed). After that, it's a parade of the lame, the halt, and the ugly.
Yes, the Undecided Voter we'd like to imagine exists, but they are like Bigfoot – rarely sighted, their existence always treated with great skepticism. I'd like to take the analogy one step further to suggest that their respective intellectual abilities are likely to be similar as well.