Firstly, the Democrats have a pretty strong buffer against Republican gains at the margins, which might be pretty useful to them since parties taking over the White House typically lose seats at the next midterm election. For example, suppose that Republicans gain 5 points across the board in 2010 (so that, for instance, a district which they lost by 3 points in 2008, they'd win by 2 points in 2010). If the Republicans managed to do this, the Democrats would lose just 15 seats, still holding 242 to the Republicans' 193. Suppose instead that the Republicans gained 10 points across the board. Surely that would give them back control of the chamber, right? Not really -- it only nets them 7 additional seats, giving them 200 to the Democrats' 235. Finally, suppose that the Republicans gained 15 points across the board. Even then, the Democrats would retain possession of the House by a narrow 219-216 margin. Put more succinctly, an outright majority of the House is now controlled by Democrats who won their elections by 15 points or more. Even if the political climate shifts back toward the Republicans, they may have trouble getting much bang for their buck.
The second advantage that comes into play is redistricting, which will take place after the 2010 census is completed. If the Democrats' voters are less efficeintly allocated now, they would seem to have more to gain once redistricting takes place and reshuffles them.
The third advantage is resource allocation. Seats that are won by 40+ points require next to nothing to defend, allowing the Democrats to concentrate their resources in more competitive areas.
Finally, there is a synergistic relationship between the vote margin in a particular district and the ideology of the congressman. That is, districts that are won by wider margins can support more progressive policymakers. The Congerssional Progressive Caucus now has 71 members, considerably larger than the Democrats' 47 Blue Dogs. Many congressional districts are so blue that the congressman is theoretically under more threat of losing to a primary challenge on his left than a Republican challenge on his right.
Although the Republicans face an arduous task in crafting a path to 270 electoral votes, finding 218 viable seats in the Congress might represent the more difficult challenge.
The work is first rate, the writing typically fluent, the thinking lucid, but at the end I found myself thinking, "So what?" OK, given the past decade, it's pretty clear that we are better off if the Republicans don't control anything more responsible than a small mayoral position -- somewhere, and God help that town -- but other than that, I'm not seeing the positive benefits of a Democratic Congress. The Blue Dogs plus the Republicans seem pretty much able to bully, bluster, and beg the majority to indulge them and not do anything "radical," and if Congress can't start driving progress, how will things get done?
Well, you say, that's Obama's job. And you're right -- it is. But it's an awful lot of responsibility to put on one guy, especially one guy of limited experience who will, if history serves as a guide, have no more than a year or so to effect meaningful change (name one major accomplishment of a president not named Roosevelt that happened in their second term). After that, the Democratic majority in Congress will be nothing more than one big firewall waiting for the Republicans to hack through. The Dems need to find a way, not just to maintain their majority, but to use it, even to expand it. Making Congress something more than a static body would also have the benefit of undoing some of the damage Bush's "imperial presidency" has done to both institutions. Congress has got to be able to lead, at least some of the time, or else it almost doesn't matter who's in charge of anything except the presidency.