Dave Wasserman of Cook Political Report tweeted this on Thursday afternoon.

Cook regularly updates its list of House races that it expects to be competitive, assigning ratings of "likely," "lean," or "toss-up" for seats by party. A "likely Democratic" seat, Cook figures, will likely be won by the Democrat. That's stronger than "lean," which means the district will probably go for the indicated party. And toss-ups are just that.

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.