Thomas Frank wrote a piece in the Wall St Journal Wednesday with that title. His answer: they embraced globalization, which enabled companies to outsource/offshore all the good union jobs, crushing the Democratic Party's core union/workingman membership.
Interesting theory, but I believe it is off the mark.
When I grew up in the South, the Democrats were clearly the party of the workingman. John Kennedy, stumping for votes in West Virginia coal mining country. Franklin Delano Roosevelt, creating the National Labor Relations Board, etc. Republicans were the party of John Lindsay, Nelson Rockefeller and Preston Bush. Wealthy country club members with Ivy League education.
So I share Mr. Frank's point of view of the origins. But not of the cause of the detour.
The Democratic leadership have converted en masse to the Religion of Environmentalim. As a result, they willingly sell out workers to (possibly) preserve some life form we would otherwise have no interest in protecting. Actually they sell out workers over the possibility that there is some lifeform at risk.
There is actually an excellent case in point in the February 8th Wall St. Journal p. A6, which describes an average wait time of seven years for approval for a new mining project. Mining jobs tend to be high-paying; commodity prices are doing well in this economy, and the country could certainly use the jobs. And mining jobs can't be off-shored.
However, the environmentalists, with their band of skilled lawyers, have learned how to delay or completely prevent many forms of development. (I won't defend the Republicans here; when they were in the majority they did little to reign in power of environmentalists and their legal counsel).
Mr. Frank: if you want to see the Democrats keep their majority position and reclaim the mantle of defender of the little man, don't worry about globalization, worry about the new religious order that has swept an old party.