From a report issued by the Republican State Leadership Committee:
Farther down-ballot, aggregated numbers show voters pulled the lever for Republicans only 49 percent of the time in congressional races, suggesting that 2012 could have been a repeat of 2008, when voters gave control of the White House and both chambers of Congress to Democrats.
But, as we see today, that was not the case. Instead, Republicans enjoy a 33-seat margin in the U.S. House seated yesterday in the 113th Congress, having endured Democratic successes atop the ticket and over one million more votes cast for Democratic House candidates than Republicans. The only analogous election in recent political history in which this aberration has taken place was immediately after reapportionment in 1972, when Democrats held a 50 seat majority in the U.S. House of Representatives while losing the presidency and the popular congressional vote by 2.6 million votes.
(via Think Progress)
The last sentence makes an important truth clear: that Democrats have a long history of gerrymandering districts in their favor as well. Redistricting is a political process, and that is not bad thing per se.
What makes the current practice of redistricting more pernicious than it has been historically, however, is that the wealth of demographic behavior available combined with modern data mining techniques makes it possible to predict voter behavior with frightening accuracy. This data is integrated into software that allows state legislatures to draw districts that are extremely reliable in their partisan allegiance. Redistricting is no longer a bunch of men smoking cigars drawing clumsy lines on a giant map on the floor. In the same way that Google is able to find the link you’re searching for out of the enormity of the Internet, so is redistricting software able to find Republican or Democratic voters with equal precision.
The New Yorker wrote about this phenomenon 10 years ago, and I recommend anyone interested in the partisanship and paralysis in Washington take the time to read it.