The shift since 2008 is hard to untangle from other forces as well, said Juliana Horowitz, the lead researcher on the project at Pew.

“It’s Obama’s election, but it’s also the recession and the post-recession era,” Ms. Horowitz said. “As we’ve seen these different types of communities become increasingly different politically, we’ve also seen them become increasingly different in their demographics and their economics.”

The abrupt shift in 2008 does suggest that growing geographic polarization is not primarily about sorting, or rural Democrats moving to the city and urban Republicans heading for the country. It’s unlikely that large numbers of partisans abruptly relocated in that moment.

Recent research by Greg Martin and Steven Webster at Emory University confirms that Democrats are more likely than Republicans to move to denser, more urban places, and Republicans to move to less dense ones. But the effect of those preferences is small — far too small to explain the level of geographic polarization we see in America today, as population density and voting results have become ever more tightly linked.

“There is sorting,” Mr. Martin said. “But this mechanism of sorting just doesn’t explain it. So then there must be something else.”

People who stay put must be changing their political views. Or the parties, as they have done historically, have changed what they stand for or how they appeal to voters.

Those two explanations, however, present another puzzle. Are politicians successfully tapping into an increasingly potent urban-rural divide among voters? Or do voters believe this divide matters because politicians (and the media) keep emphasizing it?