"That's how you know that the cause has actually become a movement," he said. "You look around and see a lot of new faces. It's no longer just the usual suspects."

What H&R describe in their paper is, in effect, the film running in reverse. Beyond a certain point, fragmentation becomes self-reinforcing. I wondered about the implications of H&R's work for what now remains the antiwar movement. Was there anything in their analysis that would suggest the possibility of its revival, on a broader basis, in the immediate future? Could it happen with Obama still in office? Or would it take the "perceived threat" of a Republican president?

I wrote Heaney to ask. His short answer was, simply, no -- the chances of a major revival in the short term are slim. The more nuanced version of his response went somewhat beyond my question, though, and seems of interest:

"As long as voters remain highly polarized along party lines," he responded by e-mail, "self-identified Democrats are unlikely to protest against Obama's policies, even if they disagree with some of them strongly. A sudden end to the era of partisan polarization seems highly unlikely. So I would say that it is a very good bet that Obama will not confront large left-wing demonstrations. Of course, LBJ faced large left-wing demonstrations, but the party system was not polarized back then in the way that it is today."

The same dynamics apply to the Tea Party: "Our analysis implies that the Tea Party will have a lower degree of organization and success in 2012 than it did in 2010. Because the Republicans won the House and made gains in the Senate, Tea Party activists feel much less threatened today than they did a year ago. So, while the Tea Party will obviously be around in 2012 -- and it will likely factor into the Republican presidential contest -- our analysis suggests that the Tea Party will not generate the same level of enthusiasm next year as it did last year."