Dean eventually collapsed basically over "electability" concerns, and we wound up with Kerry versus Edwards. Here Edwards tried to play the populist card against Kerry a bit, but didn't go too hard at it (because he wanted to be Vice President), and his own DLC associations made this a rather unlikely move anyway.
Certainly, Edwards didn't hit Kerry or any other Democrat too hard, but there was very good reason not to do that. If you use your massive populist cred to obliterate your eventual nominee's populist cred, you've given the election to the Republicans. I can't think of an Presidential election in my lifetime when the candidate coded by the media as the more populist choice lost. As it turned out, Kerry was no good at looking all populist anyway, but Edwards was right to not shoot him in the ass from the beginning. Don't attribute a weird desire for second-bananahood to Edwards when you can attribute the much more reasonable belief that gigantic risks are involved in playing hardball.
Edwards spent the late months of his primary campaign playing the hell out of the populist card -- he just didn't target any Democrats. I know you've read this before, but since people are forgetting it, I'm putting it out there again:
“Today, under George W. Bush, there are two Americas, not one: One America that does the work, another America that reaps the reward. One America that pays the taxes, another America that gets the tax breaks. One America that will do anything to leave its children a better life, another America that never has to do a thing because its children are already set for life. One America -- middle-class America - whose needs Washington has long forgotten, another America - narrow-interest America - whose every wish is Washington's command. One America that is struggling to get by, another America that can buy anything it wants, even a Congress and a President.”
Populism? Check. Revived a moribund candidacy? Check. It's this populist message that made him the last to fall. And when he fell, it was to the same electability considerations that took out Dean. As Will Saletan couldn't stop telling us, the Kerry campaign achieved victory on the backs of fools who thought the muddlemouthed Yankee who'd done something in a long-ago war was more electable than the poor boy with the best stump speech Carville had ever seen:
In Oklahoma, both Clark and Edwards beat Kerry by 13 points among "agrees with you" voters, but Kerry got away with a competitive finish by thumping them among "can defeat Bush" voters. In South Carolina, Kerry lost "agrees with you" voters to Edwards by a 2-to-1 margin but escaped with a respectable second thanks to "can defeat Bush" voters.
This isn't a rejection of populism. Even the most committed populist could vote against the populist candidate, given sufficient confusion about the electability situation. That's what took out the Edwards campaign.
When you look at the story of the Edwards run, it's a story about how left-populism remains healthy and strong.