The Democrats are losing their moderates. The few moderate Republicans like Snowe and Brown represent states with a more or less ideal Democratic voter-makeup. But they are in the minority and on the other hand Democrats are losing states where working class are abandoning them for the Republican Party. By spurning their Blue Dogs, the Democrats ensured their minority status in the House. They may yet follow suit as well in the Senate.
The dems didn’t just spurn their Blue Dogs, they used, abused and figuratively raped them to pass the far-left agenda items in 2009-2010. Rahm Emanuel recruited the so-called Blue Dogs in the 2006 and 2008 races to gain their majority. You didn’t see the dem leaders rushing to their defense in the 2010 elections.