Paul Rosenberg of OpenLeft has written a must-read about historical strains of progressivism (his pet topic) and how they correlate to Obama's politics and the current state of the party. Just read it all.

(...I'll also add that the dynamic between working-class progressivism and middle-class progressivism, which Rosenberg touches on here and has been examined extensively by educationinaction, is very real, and probably the main reason that left-populist Democrats like me are so perplexed by and suspicious of Obama.)