​

Eric Cantor's primary loss to David Brat last night was a surprise on a number of levels. For starters, it represents a prime example of the unintended consequences of redistricting gone awry. Moreover, no sitting majority leader has ever lost a primary since the office was created in 1899. And incumbents rarely lose primaries, period. Since 1968, only 130 representatives and 24 senators have lost primary contests, according to data compiled by Greg Giroux of Bloomberg news.

The data show that primary losses peak in the years following Congressional redistricting, when two incumbents can end up fighting for the same redrawn district. 1992 saw the greatest number of upsets, with 20 incumbents losing their seats to a primary challenger that year. 2012 was also a big year for primary turnover, with 7 Democrats and 7 Republicans losing their seats before the general election. So far this year only two incumbents have lost primaries - Eric Cantor, and Ralph Hall of Texas, both Republicans who were challenged from the right.

Historically, Democrats have been twice as likely to lose primaries as Republicans. They've tended to be the more fractious of the two parties, so this makes sense. In recent years, however, Democrats have been relatively unified while the GOP has dealt with internal divisions caused by the rise of the Tea Party. This has caused primary losses to even out between the parties. Since 2008, Democratic incumbents have lost 11 primary challenges, while Republicans have lost 16. All but two of those Republican losses were to Tea Party challenges from the right.

Similarly, Democrats have been twice as likely to lose primary fights due to redistricting. This has held true even in recent years. Since 2008, nearly half of Democratic primary losses (5 out of 11) have come from multiple incumbents fighting over the same redrawn district. But less than 20 percent of Republican primary losses (3 out of 16) have been due to redistricting.

This illustrates how successful Republicans have been at drawing Democratic seats out of existence. But Cantor's loss underscores the dangers of overenthusiastic gerrymandering. Virginia Republicans tweaked the boundaries of Cantor's district in 2010 to make it more conservative. This seemed like a great idea in 2012, when Cantor won his primary by a huge margin. But the unintended consequence was that the district became so conservative that it opened up Cantor to a challenge from the right, even though ideology-wise he's about as conservative as Michele Bachmann.