And here are some other general statistics, again excluding those who are retiring:

Blue Dog Coalition membership: Only 16 left from their height of 54 members in 2009 (Barber, Barrow, Bishop (GA), Bustos, Cooper, Costa, Cuellar, Gallego, Lipinski, Peterson, Rahall, Sanchez (Loretta), Schrader, Scott (GA), Sinema, Thompson (CA)).

Same-sex marriage opponents: Just 13 left! (Barrow, Bishop (GA), Cuellar, Enyart, Gallego, Green, Lipinski, Peterson, Rahall, Richmond, Sewell, Thompson (MS), Vela). (Note: The source is a bit out of date. Costa, Schrader, and Scott have since endorsed equality.)

It may appear that I'm cheering the demise of the conservative wing of the party. While replacing conservaDems in safely blue seats would obviously be helpful, there just aren't many people left like Dan Lipinski (D-IL), who persists in voting to the right of his solidly Democratic district. The unfortunate fact is that most of the Blue Dogs held deep red seats, and they've been replaced by far worse Republicans, which cost us control of the House in 2010.

The point, rather, is that this trend can't be halted or reversed. While we could hold down deeply conservative seats in the past with veteran incumbents and pick up red seats with locally popular and moderate challengers, those days are behind us. Even as we picked up seats in 2012, the conservative caucus suffered further losses, and the few new Blue Dog candidates went mostly nowhere. Matheson and McIntyre hold down the reddest seats of any Democrat, and it is not a coincidence that they are the only House Democrats to retire for reasons unrelated to health so far this cycle.

But it's not all bad news. As this phenomenon changes the makeup of the Democratic caucus, a parallel one is affecting Republicans. Witness their marquee retirements this cycle: Tom Latham (R-IA), Jon Runyan (R-NJ), Frank Wolf (R-VA), Jim Gerlach (R-PA). It's no coincidence that they are among the Republicans who occupy the bluest districts. Over the past several cycles, even during the GOP wave of 2010, Republicans have shed their most moderate members in blue seats via defeat or retirement. Their loss is our gain, and we have managed to elect mainstream liberal Democrats in their seats. These seats are also much easier to defend than Blue Dog districts, are more durable against waves, and lead to a more cohesive caucus.

The congressional maps are still severely gerrymandered against us on the whole, but with this realignment in mind, we know where the next Democratic majority will come from. And when it does arrive, it will look much more progressive than the last one.

Update: Since the publishing of this diary, four additional Democrats have joined the Blue Dog caucus, a change now reflected in the Blue Dog listing above. They are Barber, Bustos, Rahall, and Sinema. Though this does increase the size of this historically problematic group, none of them are as conservative as previous notable Blue Dogs, such as Dan Boren, Gene Taylor, Bobby Bright, and Walt Minnick. As the most conservative Democrats in the reddest seats disappear, even the Blue Dog caucus is seeing itself shift to the left.