Amid the steady (if not exactly frenzied) march of superdelegates toward Obama these last few weeks, it's worth asking what accounts for his relative success here. My favorite theory, courtesy of the New Democratic Network's super-sharp president, Simon Rosenberg, has to do with the sheer amount of time between March 4 and Pennsylvania.

One view of this interregnum held that it would be bad for Obama, since it gave the press nothing to do but scrutinize him and gave the Clinton campaign time to soften him up. In light of the Jeremiah Wright controversy, that wasn't completely wrong. But, as Rosenberg explained it to me last month, the seven-week layoff* would be even worse for Hillary. The reason: It deprived her campaign of a potential source of momentum (primary victories) and left pundits and superdelegates with nothing to chew over but delegate-math, which looks increasingly tough for her. "The difficulty of her ability to win is becoming more widely understood than it would be if we had a week-to-week horse race," Rosenberg told me. 

In retrospect, I think this is exactly right. Since March 4th, Hillary's daunting odds have become conventional wisdom. As a result, many (if not most) superdelegates now feel like the only thing she can achieve by staying in the race is to damage Obama's chances against McCain. Which is somewhat counter-intuitive, since it was Hillary who won the last big round of primaries, and who will likely win the next round.

Anyway, hats off to Rosenberg for anticipating this scenario. (And, for the record, he had the confidence to stick with it even as the Wright scandal was unfolding.)

*Obviously Wyoming and Mississippi happened after March 4. I'm limiting the analysis to constests that draw major media coverage.

--Noam Scheiber