I’ve called out a bunch of Republicans on this, but Democrats are susceptible to it too. Here’s Robert Reich:

There are two major theories about why Romney is dropping in the polls. One is Romney is a lousy candidate, unable to connect with people or make his case.

The second is that Americans are finally beginning to see how radical the GOP has become, and are repudiating it.

Well, no, there’s a third major theory: that Barack Obama should have been recognized as a mild favorite to win reelection, given the fundamentals.

Look at, for example, this Dylan Matthews compilation of forecasts from late summer, before the conventions and the subsequent Obama polling surge; it showed a close race, with Obama a mild favorite. Or, for that matter, consider the Larry Bartels model. He finds that first-year economic performance actually has a significant effect on subsequent reelection (unlike most models, which assume that voters don’t pay attention to anything that remote), but in the opposite direction: that is, the terrible economy in 2009 (not 2008 — 2009, his first year in office) should be helping Obama today. In fact, it should help him quite a bit.

Why do we care? Because setting reasonable expectations of what would be happening absent any campaign or candidate effects is the only way we can learn what each campaign is doing. Obviously, it’s hard to do that, given that political scientists disagree about exactly which “fundamental” factors affect outcomes and how they do so. But in general we have pretty good estimates available, and what’s out there leaves relatively little to explain.

That doesn’t mean campaigns don’t matter. For that matter, it’s possible that Obama is doing something that hurts him in the polls over and above the fundamentals, and that he would be losing except that Romney is doing even worse.

Still, the easiest interpretation of what’s going on right now is that, if Obama leads by 3 to 4 points, only a point or two needs to be explained beyond the fundamentals. At best, we’re talking about maybe 5 or 6 percent who would otherwise be voting for Romney but currently appear to be supporting the president.  That’s still worth studying, of course — but it’s a relatively small effect overall.

The basic story here is that, after all, it is the economy.