Quite a few commenters here on Alan Abramowitz' study of the negative impact of conservative ideology on Republican incumbent senators' general election campaigns from 2000 to 2008 wondered if he might do a counterpart analysis of Democrats over the same period.
From your lips to Alan's computer: he's done the counterpart analysis, and found that ideology does not seem to have had much impact on Democratic senatorial incumbents from 2000-2008; being more liberal didn't hurt them, all other things being equal. The numbers are clear, but the reasons for the very different partisan results are not at all obvious.
In an email exchange with Alan, I offered a couple of hypotheses which he quickly shot down. Were Democratic Senators more ideologically heterodox than Republicans, and thus more capable of adjusting to local political realities? No, in fact, Republicans had a significantly greater degree of ideological diversity than Democrats over this period. Were Democrats more ideologically suited to individual states? No, Republicans had a slight advantage here as well.
Alan thinks perhaps Republicans are more likely to be frank about ideological conservatism than Democrats are about liberalism, making their campaigns more ideological. That's another way to say that Democrats, whatever their voting records, try to "seize the center" more than Republicans.
And that leads to another factor in this analysis: Democratic incumbents did much better over this period than their Republican counterparts, losing only 4 of 67 races, while Republicans lost 17 of 69 races. It's probably helpful to remember that there was one very strong Republican cycle during these eight years (2002), and two stronger Democratic cycles (2006-2008). So it may be that ideology is more important when a candidate is already swimming against the partisan tide. That's not good news for those Democrats in this cycle with strong liberal profiles, but remember that if they are replaced by strongly conservative Republicans, the latter may have some serious problems down the road when the tides, as can suddenly happen in this turbulent era of U.S. politics, change.