The presidential election in the United States is very complex. Because of the Electoral College, it is more complicated than simply trying to figure out who the majority support. To really understand who's in the lead, one must look at the Electoral College and see who leads in the swing states.
But what constitutes a swing state. On the surface you would think that it is those states that are the closest, but this isn't really the case. If one candidate has a significant lead, then the closest states aren't the ones that determine the election, they're the ones that are going to run up the score. The swing states are the ones that determine the election if it's close. So, for example, my home state is not a swing state this year while Nevada is, even though FiveThirtyEight currently says that Missouri is closer. If Obama were to win Missouri, he almost certainly would already have enough votes to win the election, but Romney could win Nevada and still lose.
So the real swing states, the ones that are almost certainly going to determine the election, are: Florida, New Hampshire, Virginia, Ohio, Iowa, Colorado, and Nevada. But this still doesn't really tell us a whole lot. There are 22 different possible combinations, so sort of memorizing them all, knowing the swing states doesn't help in truly understanding the election. What does it mean if Obama wins Ohio but loses Florida? How much does it help Romney if he wins Colorado?
But it's possible to group the states in such a way where it's very easy to understand the significance of each state. Since they are different sizes, it's easiest to organize them into different tiers according to size. Every state in each tier has similar "properties". That is they have similar effects on the election.
So I looked at the states, and came up with three tiers. Which states are in each tier, and what they mean, is after the jump. You should be pleased to know that this will show a clear advantage for Obama.
Tier 1: Florida
That's right, Florida gets it's own tier. That's because it's the only state that can determine the election by itself. If Obama wins Florida, he wins the election. Romney could win every other swing state, but if he drops Florida, Obama remains president.
Tier 2: Ohio, Virginia
These states aren't enough to win the election for Obama by themselves, but they only need a couple other states for them to win. Assuming Obama loses Florida, a win in Ohio requires 1 extra swing state for him to win the election (except New Hampshire, in which case he ties). If he wins Virginia, he requires 2 extra swing states to win the election (Except if he only wins Colorado in addition, which would result in a tie).
Tier 3: New Hampshire, Iowa, Colorado, Nevada
Assuming Obama loses the first two tiers, he needs to sweep the third tier in order to win. Pretty simple.
This covers every possible combination of states, but is very easy to remember. You just have to remember the seven swing states and which tiers they're in.
It's actually possible to simplify it even more by looking at it from Romney's perspective. Using these tiers, you can create a checklist of what Romney must do to win the election:
1. Must win Florida
2. Must win one of Ohio and Virginia
3. Must win one of New Hampshire, Iowa, Nevada, and Colorado
If Romney fails to do one of the things in the checklist, he loses the election. But even if he checks all the boxes he may still lose the election.
When analyzing the election in this way, it's clear that Obama has a built-in advantage. Even if each swing state was 50-50, he would still win the majority of the time. That's why, in FiveThirtyEight's projections, Romney loses the Electoral College 12% of the time when he wins the popular vote, but Obama only loses 2% of the time when he wins the popular vote. (Technically, part of this is due to the fact that Obama also has a popular vote lead.)