Even though Democrats lost seats in the U.S. House in 2004, there are several indicators in that election that suggest their Party was making inroads to be more competitive with Republicans—inroads that paid off in a big way in 2006. Trends in several measures of party competitiveness were already working in the Democratic Party's favor in 2004.
First, Democrats reduced the number of House districts in which they did not field an opponent against the GOP—from 45 in 2002, to 38 in 2004, to just 10 in 2006.
Secondly, Democrats drastically reduced the number of "blow out" races in which they fielded a candidate but were on the losing end of a 50 to 99 point margin. The number of blow out races dropped from 17 in 2002, to 9 in 2004, to just 5 in 2006.
Thirdly, Democrats also reduced the number of "very uncompetitive" districts in which they lost to Republicans by 30 to 49 points: from 96 in 2002, to 87 in 2004, to just 53 in 2006.
As a consequence of fielding more candidates and reducing the number of grossly uncompetitive districts, Democrats were positioning themselves in greater number in weakly competitive and competitive races. For example, Democrats increased the number of weakly competitive races in which they lost by between 11 and 29 points: from 53 in 2002, to 81 in 2004, to 100 in 2006.
The rise in weakly competitive GOP-held districts was not a result of Democrats losing ground in competitive districts held by the GOP: competitive districts (decided by between 0 and 10 points) rose from 20 in 2002 to 35 in 2006.
By picking up several seats in 2006 Democrats will naturally have to protect several districts that they won by slim margins (28 races were decided by 10 points or less). However, the sheer number of districts that are potentially up for grabs in 2008 has skyrocketed from 2002. The Democrats have positioned themselves to make the GOP defend up to 135 districts in 2008—almost double the number of competitive and weakly competitive districts in 2002 (73).
While defending a 25-point GOP-held district may not at first blush seem like a worrisome task, the current political climate dictates otherwise. Just ask ousted Republicans like J.D. Hayworth of Arizona (AZ-05) who won by 24 points in 2002, 22 points in 2004, and then lost by 5 points in 2006—a 27 point turnaround. Or ask defeated Connecticut Congresswoman Nancy Johnson (CT-05), who lost by 12 points in 2006 after winning by 22 points in 2004—a 34-point swing.