When you see a new poll, what do you look at first? With the general election campaign nominally underway, most people would say that they look at the head-to-head matchup between President Obama and Mitt Romney.

But I’m still intensely focused on the president’s job approval numbers.

The reason has to do with my view of a presidential campaign when an incumbent is on the ballot. Based on my read of the history and the political science research on the subject, I’ve put together a rough outline of how the average voter makes up his mind. It looks something like this: