Stipulating here for a moment: the NET number of Democratic congressional retirements is -1, which is not necessarily predictive of a wave in the distance. Of the nearly two dozen Democrats and Republicans who've decided to step down, Republicans are in a good position to pick up about seven seats -- Democrats are in a position to pick up three.
But the biggest psychological driver of congressional retirements is the perception that the next election won't be worth the personal/political/ego rewards of the 2010-2011 congressional cycle. It's not so much that Democrats expect to be in the minority, although it is possible. It's that Democrats of a certain type -- John Tanner, Bart Gordon, Brian Baird -- expect to be in a narrower, more liberal majority that forces them to take harder votes -- a majority where the new Republicans elected from open district seats are suddenly invested with major bargaining powers.
Who's next? The White House worries that Rep. John Spratt, the chairman of the budget committee in the House, a long-serving Democrat from South Carolina, is thinking through the consequences of stepping down. Republicans are watching Rep. Ike Skelton (the chairman of the House Armed Services Committee) and Collin Peterson, from a devastated auto-producing district in Minnesota; if those two go, they expect a half dozen other Democrats to take their lead.
The GOP needs 41 pick-ups to retake the House.