The House of Representatives consists of 435 members who represent proportional electoral districts of about 700,000 constituents each. Most state legislatures set those district boundaries. Notably, redistricting does not affect US Senate elections, which are statewide. Partisan state legislatures continue to employ redistricting in the hopes that it will send members of their preferred party to Congress. State legislatures also redraw districts to favor incumbents. According to a May 2005 report [PDF] published by The Reform Institute, only five incumbent members running for reelection in 2004 were defeated. Out of 435 seats, 401 Representatives sought re-election and 99% retained their seats. There is debate that, over time, Republican-controlled state legislatures have more frequently engineered outcomes where the number of elected Representatives does not reflect the popular vote.

Republicans in the House of Representatives currently hold a majority, even though they received fewer total votes nationally than House Democrats in the 2012 election and political observers have attributed this result to redistricting. Election data show that in most states where Republicans controlled the redistricting process, Democrats received a low proportion of House seats when compared to the percentage of votes for US President Barack Obama. This implies that Democratic votes are being undercounted in Congressional elections due to redistricting.

Some state legislatures directly passed legislation defining new congressional district boundaries after the 2010 Census. In May 2011, Texas passed a bill redefining [PDF] congressional districts. The state’s redistricting plans have been subject to ongoing legal challenges. In December 2011, Pennsylvania’s state legislature also passed a redistricting bill.

Even though some state legislatures were quick to pass redistricting measures, other states have employed different approaches. For example, in Iowa, the nonpartisan Legislative Services Agency draws [PDF] neutral, population-based districts after every census. In California, voters approved Proposition 20, which created a redistricting commission tasked with neutrally redrawing congressional districts.