From your writing and that of other political scientists I've absorbed two main messages about presidential elections. 1) There are few true independents; most people have a clear partisan voting record, including many who identify as "independent." 2) A dominant factor in determining the outcome of a presidential election is the state of the economy. My question is, in a political system marked by strongly partisan voting behavior, where does the flexibility to respond to the economy come from? Do the few "true independents" vote almost purely in the direction of the fundamentals and swing the election by themselves? Do the partisans of the party facing bad fundamentals not show up to vote? Is the action mostly with a large number of "weak partisans?" Something else? All of the above?That's a good question. I'll take a shot at it, but if anyone is lurking around here who is more expert about voters than I am, I welcome additions/corrections/better explanations.
First, the quick tour of the numbers. The rule of thumb here is that the electorate is equally split three ways between Democrats, Republicans, and those who say that they are independents, and that the latter group is split three ways between functional Democrats, functional Republicans, and real independents. I just like to repeat that formulation whenever I get a chance.
Okay, on to the question. The way to think about it, I think, is in terms of multivariate probability at the individual level. Suppose we think of a voter, and assume to begin with that she has a 50/50 chance of voting for either candidate. If she has a partisan attachment, that's going to make her far more likely to vote in the direction of her partisanship (with that number being bigger for strong partisans than for weak partisans). Then there's her evaluation of the economy and, more generally, of the in-party's record; that's also going to have a good-sized push on her vote decision. And then there's everything else: to the extent that we can separate them from party and retrospective judgement, there's her feelings about the candidates, and specific issues, and her reactions to campaign materials, and whatever else. Those things may have real, but very small, affects. Add up each of those effects, and you wind up with her vote. You can also separate out the "retrospective effect," if you like, at least conceptually -- the biggest part of it will usually be the economy, but there might be real if small bits for evaluation of foreign policy, or allowing major US cities to drown, or whatever.
So, yeah, a lot of what's happening is going to be among true independents and weak partisans, because they're closer to 50/50 after factoring that in. And, then, it's certainly possible that turnout could play a part, too -- but generally remember that the hardest partisans are also generally going to be the ones most likely to vote.