Wasserman's tweet implies that the downward trend cited -- 100 competitive seats in 2010; 57 in 2012 -- is perhaps inevitable. And further that it is perhaps linked to "The Big Sort," a trend in which people move to neighborhoods that share a political sentiment, thereby making elections less generally competitive.
Going back to 2010, though, gives a distorted view of electoral changes. Wasserman's included only "lean" and "toss-up" races in his estimates, so we pulled those numbers for each of the last seven election cycles in the last week before the final vote.
2014 is in line with 2004, just as 2012 was in line with 2006. And nothing is in line with 2010, a massive wave election following two elections in which Democrats picked up seats that they might not have been expected to win. You can see that on the graph below.
There are certainly fewer contested races this year than in recent years, but it's hardly unprecedented. 2010 was an outlier. Which is not to say that Wasserman's point is wrong, necessarily. If the Big Sort theory is correct, and if other factors (like gerrymandering) continue to move House seats away from being competitive, we'd expect to see a continues drop-off over time.
It's just that the time period needs to be more than four years.