Paul Waldman, over at TAPPED, has a post so good, so on the money, that it deserves to be quoted at length.
THE DIXIE BONUS. Ezra notes below, "Edwards' Southern accent and manners are critical in his ability to project a much more combative, sharp form of liberalism than the others are offering. What would sound like Marxism from the mouth of Howard Dean or Hillary Clinton sounds like good, old-fashioned, American populism from Edwards."
And why is Edwards at least partly right that he can go to places where Clinton, and to an extent Obama, can't?
Part of this is that, to be frank, while people in Rhode Island or Oregon don't look on presidential candidates who come from regions other than their own with suspicion, lots of southerners seem to be reluctant to vote for people who don't share their drawl. Of course, this is never characterized as pathological regional xenophobia -- it's just how regular folks think, and there's not supposed to be anything wrong with it.
Southern-ness, furthermore, is supposed to be a marker of "authenticity." People who are from the South are genuine, forthright, the kind of folks you'd like to have a beer with, while if you come from somewhere else, chances are you're a big phony. Witness Fred Thompson, the "down-home" corporate lobbyist. Southerners are always taking offense at people who supposedly look down on them, but to someone who was raised in the Northeast, the idea that southerners are inherently more "real," and more American, than the rest of us is deeply insulting.