Last week, the big story in the presidential race was Mitt Romney's momentum. "It’s momentum vs. the map," wrote Politico on Oct. 22.
Romney was up by 0.4 percentage points in the Real Clear Politics average of head-to-head polls that day, with an average of 47.6 percent of voters saying they favored him against President Obama. The next day, he was up by 1 percentage point, with 48 percent of voters saying they preferred him in the presidential race. That would prove to be his peak.
Friday, Romney is trailing by 0.3 percentage points, with 47.2 percent of voters saying they favor his candidacy. That means that at pretty much the exact moment when the media began talking up Romney's momentum, Obama was gaining ground.
There's an argument out there that the idea of "momentum" is largely a conceptual error. Momentum means something in physics: A car rolling down the hill gathers momentum as a result of it rolling down the hill. But that's not obviously true in politics. There's no reason to think that a candidate experiencing a few good days of polls or news will, by virtue of that good news or good polling, experience more good news and good polling.
But I'd go further: I'd bet that a careful study of media mentions of a candidate's "momentum" would find that they tend to presage that candidate losing altitude in the polls. That's because while "momentum" may not be real, reversion to the mean is.
When a candidate has been overperforming where they've been in the race for long enough that the media has become convinced of their "momentum," that's good evidence that they're getting an unusually good run of good news or good polls, and the race has drifted away from its fundamentals. And when that happens, it tends to mean that the likeliest thing the race will do next is return to its fundamentals.
In September, for instance, President Obama gained a big lead in the polls, partly in response to a strong convention and Romney's 47 percent comments. Suddenly, a race that was looking very tight began to look very lopsided. But Obama's good run didn't last. After the first debate, that lead evaporated, and Obama fell significantly behind in the polls. But Romney's good run didn't last, and now we're back to where we've been for most of the campaign: A close race with a slight but durable Obama lead in the swing states. Today's race looks a lot like August's race -- before the two candidate had their turns on "momentum" roller coaster
"Momentum" narratives, in other words, tend to signal that the race is somehow detaching from the fundamentals. Sometimes, there's reason to think the change will persist. But in most cases, the safe bet is probably that whatever is causing the "momentum" will end, and the race will drift back to pretty much where it's been.