have southern democrats become, by default

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Well, they still have Manchin who is pretty DINOish, as opposed to merely being a conservative Democrat like Hagen. Zell Miller was crazy right-wing his last couple of years in the Senate ending with 2004. He was more liberal in the 1990s.

Of course it helps that right-wingers like the founders of the Blue Dogs stopped pretending they were Democrats.

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The premise of the Blue Dogs was that you could have conservative Democrats in Republican-voting districts who would work with Republicans on many issues. Since the advent of the Tea Party aka the "No Compromise" wing of the GOP and the defeat of half the Blue Dog Coalition in the 2010 midterms, that strategy has been proven to have been foolish.

So why was Ellen Tauscher a Blue Dog in a D+11 district? There are still plenty of Democrats who are far more conservative than their district would allow, such as Dan Lipinski or Tom Carper.