On the American Prospect site, Mark Schmitt today offers a fascinating analysis on the most fundamental differentiation among the Big Three Democratic presidential candidates:

This is not a primary about ideological differences, or electability, but rather one about a difference in candidates' implicit assumptions about the current circumstance and how the levers of power can be used to get the country back on track. It's the first "theory of change" primary I can think of.

Hillary Clinton's stump speech is built around the speechwriter's rule of three, applied to theories of change: one candidate believes you achieve change by "demanding" it, another thinks you "hope for it," while she alone knows that you have to "work for it."

That's accurate as a rendering of the candidates' language: Her message of experience and hard work, Obama's language of hope and common purpose, Edwards' insistence that those with power will never give it up willingly.

Schmitt goes on to defend Obama's own "theory of change," suggesting that only a "common purpose" approach can build the political capital necessary to defeat conservatives and special interests and deliver real change. But whether you agree with him about Obama or not, Schmitt does nicely define the battleground which the candidates have chosen.