Before Obama, all but five of the last six elected presidents — the exception was Jimmy Carter — had a House controlled by the opposition party at some point during their tenure.
But it's rare for one party to win more House seats while securing fewer votes than the other party. The last time it happened before 2012 was in 1996, when Democrats won the nationwide House vote by 43.6 million to 43.4 million as Republicans held their majority and Bill Clinton was re-elected president, according to the House Clerk's office.
The variance between the nationwide partisan vote total for House candidates and the Republican-Democratic seat split in the chamber is caused by demographics and redistricting, the once-a- decade redrawing of political boundaries that Republican- controlled statehouses dominated following the 2010 census.
Redistricting is intended to ensure House members represent roughly equal size populations. Yet from the first Congress, party leaders began exploiting the map-making exercise by weakening the voting strength of some groups to gain partisan advantage, a practice known as gerrymandering.
Urban cores are packed with black, Hispanic and single white voters who, exit polls show, voted strongly Democratic in 2012. As of 2011, half of all Hispanics were concentrated in 30 of the nation's more than 3,100 counties; half of all blacks lived in 60 counties, said William Frey, a senior fellow at the Brookings Institution's Metropolitan Policy Program in Washington.
When Republicans control redistricting, they can pack Democrats into a limited number of districts while maximizing, with computer precision, their party's advantage in other House races.
State lawmakers can build databases with detailed voter registration figures, election results and population data to project campaign outcomes and demographic trends. It's also easier to predict voting patterns since fewer than 30 districts backed the presidential candidate of one party and a House candidate of the opposite party in 2012 — the lowest rate in at least 90 years, according to data compiled by Bloomberg.