From the WP:

In a year of bad omens for the GOP, the latest batch of disclosure forms filed with the Federal Election Commission offers one more: Incumbency no longer means that embattled Republican representatives can expect to overwhelm weakly funded Democratic challengers with massive spending on advertising and get-out-the-vote efforts.

There are 27 Republican incumbents classified by the nonpartisan Cook Political Report as the most vulnerable to losing reelection this fall. These incumbents still boast a clear fundraising edge, but it is much less pronounced than in years past. According to calculations made from FEC data, the Democratic challengers in these races have raised about 60 percent of what their opponents have collected and have about the same percentage of cash on hand.

At this point in the 2004 election cycle, by contrast, Cook listed nine Republican incumbents as similarly vulnerable. Their Democratic opponents had been able to raise 42 percent of what their opponents collected, and challengers' cash on hand was a lower percentage. There were similar disparities in the 2002 cycle.