Part of it is that a voter can feel badly pulled between different issues, and unsure which is most important. She (by the polls I’ve seen, the undecided voter is more likely to be a woman) might be pro-choice, which would make her lean toward President Obama — but unemployed, which may lean her toward Mitt Romney.

Her abortion position, usually a reliable indicator of her vote, has been overtaken by more pressing economic concerns. And thus an undecided voter is born.

Another truth: Many undecided voters weren’t actually undecided all along. For example, a September Quinnipiac University/CBS News poll had Obama leading Romney in Ohio by 10 points; now a new Quinnipiac University/CBS News poll has that lead down to five. Yet both polls had 3 percent undecided. Do you really think they’re the same people — still dithering, while 5 percent have switched sides? Or were some of Romney’s new 5 percent undecided last month — while some of the 5 percent who left Obama are now the new undecided?

Then there’s the “bandwagon effect”: Lots of people, no matter their affiliation, go with the candidate they think will ultimately win.