And in the 1912 election, it was clear where the voter split occurred. Maybe I'm just being obtuse, but I see a lot of 2008 Obama supporters who aren't impressed with the last 4 years. I also personally know people who normally vote third party who took a chance with Obama in 2008 and learned their lesson. I see Johnson picking up a lot of former Obama supporters. I don't see Obama keeping the support he had, because he didn't live up to a lot of "special interest" expectations. For instance, the Libertarian party probably looks pretty attractive to a 2008 supporter disgusted by Obama's lackluster, Johnny-come-lately stance on Gay and Lesbian rights.
Then, on the other side of the coin, the Republicans themselves claimed for much of the primary season that the Paul supporters weren't "real" Republicans. Supposedly they were all Libertarians who had "infiltrated" "legitimate" GOP events to support their "radical" candidate. That's a lot of third party support that the Republicans never really had to begin with.
What I'm saying is, this time the "voter split" is going to happen on both sides of the aisle. I could easily see the general election splitting roughly into thirds. (Well, realistically more like 40/40/10, but the point still stands.)
Edit: And if the popular vote did split into thirds, what would the electoral college do? If they can't determine a clear winner, doesn't it go to the (Majority Republican) House of Representatives to decide? If they can't decide, wouldn't that put Biden into office? Seems like it could be a very interesting election, if people allow themselves to break from their self-imposed servitude. Or we could have the same old same, and continue the slow march along the path the Romans laid out for us.