Charlie Cook says that Republicans are getting sucked under in his National Journal column (subscription required):
By virtually any measure, the Republican Party's national poll numbers are at least as bad as Democrats' were before their 1994 debacle. For a time, the GOP's national problems did not seem to be spilling over onto individual Republican candidates. But since the first of the year, we have begun to see evidence that most House Republicans are running 5 to 10 points behind where they would be in the absence of a national undertow. And Republicans aren't being pulled down just in those few races where Democrats are fielding first-tier challengers. The pattern now extends to contests where the Democratic candidates are mediocre at best. One GOP strategist cautioned, however, that many GOP incumbents may have become complacent because they've won in recent years by margins that were bigger than they should have been, given their districts' political makeup. And strategists in both parties suspect that some of the GOP incumbents most likely to end up losing are those who have not had difficult races in many years, if ever, and may be resistant to doing what it takes to wage a strong campaign.
Unfortunately, there are few reliable independent polls conducted in House races. Political reporters and analysts are being deluged with polls conducted for Democratic candidates that tout their gains. And Republican campaigns are releasing few poll results that contradict them. GOP pollsters, media consultants, and other operatives privately concede that the drumbeat of bad numbers coming in -- but not publicly released -- has become depressing and alarming. What most concerns GOP strategists is that these congressional polls may not fully capture the extent of their party's problems, because other polling shows that Republican voters are disillusioned about the Iraq war, the federal budget deficit, illegal immigration, port security, gasoline prices, the response to Hurricane Katrina, and so on -- and are showing less interest than Democrats in the November election.