We’re at a historic moment — Barack Obama’s estimated lead in the national polls is over seven percentage points:

There are two ways to interpret what’s happened. Most journalists will soon converge on some narrative where John McCain has lost ground due to some combination of the financial crisis, the incoherence of Sarah Palin’s TV interviews, and McCain’s allegedly “small and angry” debate performance.

For instance, The Atlantic’s James Fallows, who previously made the unsupported claim that “moments” from televised general-election debates have “figured in the ultimate outcome” in the presidential elections of 1960, 1976, 1980, 1984, 1988, 1992, 2000, and 2004, now refers to John McCain’s “sourness and anger” during the first debate as a “moment” that ‘”mattered’”:

“Everyone knows,” based on a long string of past episodes, that some unintentional flash of character revelation usually turns out to be the memorable aspect of a presidential debate. Eg: Nixon looking furtive and sweaty in 1960, Ford momentarily seemed befuddled in 1976, Dukakis seeming bloodless in 1988, etc. All these moments “mattered” because they crystallized a feeling that, in retrospect, people knew they’d “always had” about the candidate.

In the days since the first Obama-McCain debate, it’s become ever clearer that John McCain’s sourness and anger are the traits unintentionally revealed in the debate and now working against him… Thanks to McCain’s hostile refusal to engage Obama as a human equal face-to-face at the debate, the image he is cementing is that of a seething older man. Like Bob Dole in 1996, with less of a gift for one-liners.

It all fits into a pattern in retrospect — but I don’t know a single “expert” who predicted that avoiding eye contact would be the enduring image of the first debate.

The key phrase is “It all fits into a pattern of retrospect,” which is precisely the point — humans are quite good at constructing post hoc narratives to explain observed events. Princeton’s Larry Bartels recently recounted an amusing anecdote along these lines:

I was very struck when I learned — many of you probably have seen, after each recent election, immediately after the election, Newsweek comes out with a big cover package on why, fill in the blank, won the election. And in 2004, they actually came out with a book that included a lot of analysis of why it was that Bush won the election. But before the election, they actually sent out an advertisement that had two books side by side; one was why Bush won the election and the other was why Kerry won the election. And given the times of producing these things, they actually had to produce most of the package, explaining to the readers of Newsweek the following week why it was that Kerry won the election.

Now, I didn’t read that issue, but I’m pretty sure that if I had read that issue, the narrative of how it was that Kerry had won the election would have been about as convincing as the narrative of how it was that Bush won the election.

So after the fact, it’s really easy to come up with explanations based on particular events, but to know whether those were the events that actually drove the usually pretty small deviations from the underlying fundamentals that we observe in a particular campaign is quite difficult.

As Bartels’s answer suggests, I think a better explanation is that the underlying political fundamentals are reasserting themselves — the dynamics of this year are simply remarkably unfavorable to Republicans. This account is consistent with Andrew Gelman and Gary King’s research showing how voters tend to converge to predicted levels of support for the presidential candidates over the course of the campaign.

Looking back, there’s a similar divide over 2004, which is frequently framed as a story in which Karl Rove’s tactical mastery and the Swift Boat ads triumph over John Kerry and his ineffective campaign tactics. The problem for Kerry, however, was that he was always fighting uphill. Political science models of the election published in the journal PS before the election had a median forecast of 53.8% for President Bush’s two-party vote:

Now, it is true that Bush’s numbers bounced around over the course of the campaign as various events occurred, as Charles Franklin’s plot below illustrates:

We can certainly tell stories about those events (such as the Swift Boat ads). However, the ultimate outcome was relatively close to the projection — Bush ended up winning as all the models projected (except for one projected statistical tie of 49.9%) and received 51.2% of the two-party vote.

This year, the comparable set of forecasts in PS, which was just published, shows a median two-party vote forecast for John McCain of 48%, which corresponds to 52% for Obama. Only one model projects that McCain will win:

If you include the projections from Ray Fair and Douglas Hibbs of Obama receiving 51.5% and 52% of the two-party vote, respectively, that I’ve previously highlighted, the median forecast remains Obama 52%, McCain 48%.

Obama’s lead now is consistent with those projections (he’s currently at 53.9% of the two-party vote in the Pollster.com estimate above). In fact, despite my previous warnings that Obama may underperform due to the influence of race, he is currently in a very strong position — the fundamentals favor him and he has a lead with approximately one month to go. Despite media hype about “October surprises,” these these plots of recent competitive presidential campaigns from James Stimson’s Tides of Consent illustrate that there are rarely dramatic shifts in the polls this late in the game:

That’s why I expect John McCain to start taking more risks in the next couple of weeks, particularly at the next debate. Expect to start hearing a lot about Tony Rezko, Jeremiah Wright, and William Ayres. They’re the three cards that McCain has yet to play.

Update 10/5 10:40 PM: Matthew Yglesias relates an anecdote that mirrors the one from Bartels above:

My mother worked in the pre-Photoshop version of Newsweek’s art department so I saw as a kid their “Dukakis Wins!” complete with a banner teasing their account of how he did it and why the prognosticators were all wrong. As Bartels says, most of that account would have to have been written in advance.

Update 10/13 10:37 PM: Kevin Drum makes a similar point.