Diverging numbers in the polls: What's the real cause?

The Impact of the ‘47 Percent’

By NATE SILVER

After a secretly recorded videotape was released on Sept. 17 showing Mitt Romney making unflattering comments about the “47 percent” of Americans who he said had become dependent on government benefits, I suggested on Twitter that the political impact of the comments could easily be overstated.

“Ninety percent of ‘game-changing’ gaffes are less important in retrospect than they seem in the moment,” I wrote.

But was this one of the exceptional cases? A week and a half has passed since Mr. Romney’s remarks became known to the public — meaning that there’s been enough time to evaluate their effect on the polls.There’s a case to be made that they did damage Mr. Romney’s standing some.

In the chart below, I’ve tracked the progress of the national popular vote in the FiveThirtyEight “now-cast” over the past five weeks. The “now-cast” reflects our best estimate of what would happen in an election held today, based on a combination of recent national and state polls. Unlike our Nov. 6 forecast, the “now-cast” does not account for economic measures, and it does not adjust for the effect of the party conventions. This makes it a little bit more straightforward to interpret in terms of tracking the progress of the polls in real time.

(More here.)