Challengers, challengers, challengers, we got 'em. You can't have a wave election without lots of viable opposition candidates to wipe out vulnerable incumbents. This cycle, it looks like the Democrats have them in spades. Michael Malbin at Brookings:
Record numbers of Democrats continue to emerge who are willing to defy the normal odds by mounting early challenges to sitting House incumbents. As of June 30, we counted 209 Democratic challengers who had raised $5,000 or more. Three months later, the 209 had grown to 391 Democratic challengers with at least $5,000. Of these 391, 210 raised at least $50,000 and 145 raised at least $100,000…. [T]here were many more Democratic challengers at the end of September 2017 (at each financial level) than for any other party cohort since 2003. The Republican class of 2009 was in second place, with all others well behind. At each level, the GOP class of 2009 had fewer than half as many challengers as the Democrats of 2017….
A necessary, if not sufficient, condition![M]any more Republican incumbents are facing Democratic challengers at this time of the year (at each funding level) than any other set of incumbents since 2003. Because multiple candidates are in fact running in some districts, the advantage is not quite as large as it was for the raw number of challengers. There were more than twice as many Democratic challengers in 2017 as Republicans in 2009. When we flip the question around, there are just short of one-and-a-half times as many Republican incumbents facing Democratic challengers in 2017 (at the various funding levels) as the other way around in 2009. While not as large an advantage as the raw number of challengers suggests, this is still considerable. The 2009 GOP cohort was larger than any other before this year. It was large enough to feed a major partisan shift in 2010—and larger than the Democrats need to become a majority in 2018.