So what is the difference between the candidates? One of the most interesting pieces of analysis previewing the primary contests has been Mark Schmitt's essay in the American Prospect on the 'theory of change primary'.
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.
But Schmitt goes on to offer a deeper analysis of Obama's pitch - and helps to explain how Obama manages to reconcile being probably the most conventionally 'liberal' of the major Democrats seem with his bipartisan appeal to independents and Republicans.
His piece has been much praised by commentators and bloggers. While this is a little late, this seems a good moment to link to it, just before we begin to find out which theory chimes most with the voters.