Republicans certainly seemed a bit rattled after they lost their bid to retain a seat that they had previously held in Mississippi's first congressional district. Part of the reason for being so worried is that the district they lost on Tuesday was not just one that they had previously held, but also one that Bush had carried with over 62% of the vote in 2004. But was this just an outlier, or a continuation of what happened with Republicans in 2006?
The figure below plots each congressional district contested in 2006 based on the percentage of the vote Bush won in the district in 2004 and the percentage of the vote the Republican candidate won in the district in 2006. The plot is divided into quadrants. Districts in the top left quadrant are those where the Republican House candidate won a majority despite the fact that Bush did not carry the district in 2004; districts in the bottom right are those where Republican candidates did not win a majority of the vote despite the fact that Bush did carry the district in 2004.
Note that very few districts fall in the top quadrant as very few Republicans over-performed in 2006. Far more of the districts fell in the lower right quadrant with many Republican candidates losing districts that Bush carried in 2006. I added the Mississippi special election outcome to this plot to give some perspective of the extent to which this result followed what happened in 2006. The district also falls in the lower right hand quadrant, in close proximity to dozens of other districts where Republican candidates under-performed.
Of course, many of those districts in the lower right-hand quadrant are those where Democratic incumbents were running, therefore making it more difficult for the Republican candidates. Therefore, it is also instructive to look just at open seat contests, which the figure below does.
Note that when it came to open seats, not a single Republican candidate carried a district that Bush did not win in 2004. On the other hand, more than a half-dozen districts were won by Democrats in 2006 despite the fact that Bush had carried those districts two years earlier. The Mississippi special election, which was also an open seat contest, stands out as a bit more of an outlier here. There were only three open seat contests in 2006 where the Republican candidate under-performed by as much as Greg Davis did in the Mississippi 1st district this week.
Thus, the special election isn't a dramatic outlier when compared to other House elections in 2006. However, the result does suggest that prospects are not improving for Republicans; indeed, they may very well be getting worse. This is particularly true when you consider that Republicans are going to be defending a lot more open seats in 2008 than they were in 2004.
For the most recent ranking of House races by the Cook Political Report, check out this helpful chart.