Not-So-Big Mo

Unfortunately, I’ve heard nothing but crickets in response to my request the other day for evidence that media horse-race perceptions of “the lead” or “momentum” in a close presidential race affect actual voters. But Nate Silver does offer some valuable thoughts today on the inherent implausibility of the “momentum” idea, along with the weak support he finds for the idea Mitt Romney’s got it:

The term “momentum” is used very often in political coverage — but reporters and analysts seldom pause to consider what it means.

Let me tell you what I think it ought to mean: that a body in motion tends to stay in motion. That is, it ought to imply that a candidate is gaining ground in the race — and, furthermore, that he is likely to continue to gain ground.

As a thesis or prediction about how polls behave, this notion is a bit dubious, especially in general elections. In races for the United States Senate, for instance, my research suggests that a candidate who gains ground in the polls in one month (say, from August to September) is no more likely to do so during the next one (from September to October). If anything, the candidate who gains ground in the polls in one month may be more likely to lose ground the next time around.

This last insight is based on the mathematical concept of mean reversion, which in politics suggests that the “fundamentals” that produce the “natural” state of any given contest tend over time to override short-term trends. Barring large external events (and those who think the first presidential debate was one of those are basing their judgments on some combination of spin and polls and spinning polls), and particularly at a time when partisan preferences are so fixed and powerful, it’s really implausible to suggest that suddenly one presidential candidate or another has taken flight and is now unstoppably building an ever-greater lead–because of “momentum!” And according to Nate, there’s little or no evidence that’s happening anyway:

Part of the confusion (and part of the reason behind the perception that Mr. Romney is still gaining ground in the race) may be because of the headlines that accompany polls.

We’re still getting some polls trickling in where the most recent comparison is to a poll conducted before the Denver debate. We should expect Mr. Romney to gain ground relative to a poll conducted before Denver. (Mr. Romney may have lost a point or so off his bounce, but he has clearly not lost all of it). But it isn’t news when he does; Mr. Romney’s Denver gains had long ago become apparent, and priced into the various polling averages and forecast models.

This should be part of anyone’s mental furniture by now, but it bears repeating: when looking at polls for “trends,” you have to look at who’s doing the polling, when the poll was taken, and what relationship it has to other polls done in the same place at the same time. Anyone can stack up a list of national or state polls to build a tenuous case for steady Romney gains, but often that involves confusing release dates with polling dates, or simply dismissing contrary data. “Momentum” is usually in the eyes of the spinner, and its power is usually derived from the panicked reactions of the “other side,” which fears (without evidence) that “momentum” turns votes. I don’t see it, but the dynamic explains a lot of what we are hearing from the commentariat this week.

UPDATE: At WaPo, Jonathan Bernstein is even more categorical: “In thinking about elections, the idea of momentum is useless.”

Ed Kilgore

Ed Kilgore, a Monthly contributing editor, is a columnist for the Daily Intelligencer, New York magazine’s politics blog, and the managing editor for the Democratic Strategist.