What I find interesting about this whole conversation is the lack of focus on actual Congressional districts. When you look at the 84 CD's currently held by Democrats, that went for either Bush 2004 or McCain 2008, the 48 Democratic seats that went for Bush and McCain, the 54 seats that were in Republican hands four years go, it is very clear that the party's vulnerability exceeds their margin of 40 seats.
In particular, Cook says, the remaining Southern Democrats who hold competitive seats are vulnerable:
I was interested in your comment, "There's no comparable regional partisan shift working against the Democrats right now."
Have you been in the South lately? The level of anti-Obama, anti-Democratic and anti-Congress venom is extraordinary, and with 59 Democrat-held seats in the region, 22 in or potentially in competitive districts, this is a very serious situation for Democrats. I have had several Democratic members from the region say the atmosphere is as bad or worse than it was in 1994.
This is not just about President Obama. It is anti-Congress and anti-Democratic Congress.
While the election is obviously 13 months away and much can change, that means it could get better, or snowball and get worse. To the extent that Democratic performance in 2008 was elevated by unusually high African-American turnout, that exposure to decline is even greater.
At this point, Democratic members in the South, Border South, Mountain states, in districts with heavy rural and small town populations as opposed to urban and suburban, particularly those with few transplants from other parts of the country, and fewer college graduates, are at particular exposure. Some of these members have either never had a tough race or haven't in many years, with campaign organizations that are hardly sharped to a fine edge.
So while the Democratic performance in the generic Congressional, which is substantially lower than it was during the periods leading up to the 2006 and 2008 elections, when these majorities were built, that is only part of the case for why this may be an extremely challenging election for Democrats.
How seriously should we take these objections? On the first point, Cook's job is to focus on the details of individual races, so it's not surprising that he thinks we should do so. But it's easy to be drawn into highly idiosyncratic narratives and end up losing sight of the big picture. In particular, individual House races are a noisy, lagging indicator of national trends (see, for instance, the House races that suddenly became competitive very late in 1994 and 2006). Political scientists try to abstract away from these details and analyze the underlying process that generates House election outcomes. Cook argues that many House seats held by Democrats are potentially vulnerable, but majority parties always hold marginal seats. The question is whether the number of potentially vulnerable Democratic members is significantly greater than, say, the number of vulnerable Republicans in 2006. (In technical terms, what does the seats-votes curve look like for 2010 relative to previous elections?)
In terms of Cook's second point about the South, I'm open to the idea that the regional shift against Democrats is not complete, making some members there particularly vulnerable. But with only 22 in competitive or potentially competitive races, it's not clear that enough Southern Democrats will lose to create a 1994-style landslide.
Update 10/5 9:20 AM: See my post comparing the GOP image across the last five midterm election cycles for more on the 1994 comparison. I also considered regional differences in a second post, which did not find large differences in GOP favorability between the South and other regions in 1993-1994 or 2009.