The trickle of retirements for House Democrats has become a flood. Bart Gordon, a 13-term Blue Dog from Tennessee, has decided to retire.

Rep. Bart Gordon (D-TN) will retire after 13 terms, he announced today in a press release, becoming the 4th Dem in a potentially competitive district to step down in the past 4 weeks [...]

But regardless of the reason, Gordon leaves open a seat Dems could have trouble defending. Sen. John McCain (R-AZ) won the district in ’08 by a 62%-37% margin, and national GOPers instantly characterized it as a lean-GOP seat that is likely to eventually fall into their hands.

Gordon joins Reps. John Tanner (D-TN), Dennis Moore (D-KS) and Brian Baird (D-WA), all of whom have announced they will step down since Thanksgiving. GOPers are thrilled with their prospects in all 3 seats.

Losing any of these members from the majoritarian House doesn’t matter all that much, and none of them are really the kind of Democrats that help to pass a decent agenda, anyway. This matters for only two reasons:

1) This is the fourth Democrat to retire in the past month, and the trend is toward more and more open seats in swing districts or (in this case) flat-out Republican territory. That means more seats to defend, less money to protect the incumbents that stay around, even less money for whatever challengers emerge, etc. This has a snowball effect on the elections, and with every retirement, the chances of major losses for Democrats goes up marginally.

2) With each retirement we are seeing a weakening of the Blue Dog Caucus in the House of Representatives. The national Democratic committees may try to spend millions to keep other Blue Dogs in marginal districts, but Blue Dog fundraising has been pathetic recently, and that could be motivating the exodus. A loss of Blue Dog power actually improves the chances of progressive legislation in the House, provided that they maintain the majority, of course. Combine these retirements with the Blue Dog seats threatened by Republicans, and the Democratic primaries of Blue Dogs in more liberal districts that could yield victories, and you have a severe contraction of a caucus that has had an outsized influence on legislation in recent years.