I've seen a lot of articles like this one in Newsweek trying to explain and/or predict the election. This particular one mentions stem cells, gay marriage, the right to die and abortion as "sleeper" issues that might be big factors. It doesn't mention illegal immigration, and most of the other analyses are strangely silent on the subject as well.
I think illegal immigration is the sleeper issues of all sleeper issues. The politicians have been almost oblivious to the seething anger in many ordinary Americans about it, and the press seems mostly clueless, too. I've talked to at least half a dozen people in this building today for whom that is THE issue in the election. What makes people more angry than anything is that they see almost the complete breakdown of political institutions on the issue, and they don't feel they can trust anybody to talk about it honestly and do what they say they will. They hear from the left about how "unfair" America is to the illegals already here. They hear from the right about the economic importance of the "guests" (i.e., cheap labor). Who is left to talk plainly about what a mess we continue to make of how to deal with being the one propserous country with the world's longest border with a third-world country?