Once every 10 years, after each U.S. Census, states must redraw political boundaries to reflect demographic changes, a process called redistricting. Districts must have equal populations and should not dilute minorities' voting powers by splitting their vote. The process can become highly politicized, with parties jockeying to draw favorable districts and keep incumbents in office. Legal challenges to new districts drawn by the Nevada and Colorado legislatures, for instance, went to the states' Supreme Courts last year, where rulings mostly favored maps drawn by Democrats.
In an attempt to depoliticize things, California implemented a new redistricting process after the 2010 Census, creating a 14-member independent commission that excluded politicians and lobbyists.
The new process, coupled with California's new primary system, which advances the top two candidates to the general election regardless of party, means that this year, the state's congressional races may be some of the most competitive nationwide. Consider the contest between two Democratic incumbents, Howard Berman and Brad Sherman. Historically, a large Latino segment of Los Angeles' San Fernando Valley was divided in a way that allowed the two to split their vote: Berman, a 30-year veteran, captured District 28, and Sherman, who's served 15 years, controlled District 27. The oddly drawn districts, known as the horseshoe and the stake, effectively kept a Latino candidate from successfully challenging either incumbent.
Now, however, those Latino voters are reunited in the newly drawn District 29, where Democrat Tony Cardenas and Independent David R. Hernandez are facing off. Both Berman and Sherman were drawn into District 30, and are competing in a tight, expensive race.
Independent redistricting commissions are not a cure-all, though. Arizona has one, and last year, Republicans displeased with draft maps that made some of their historically safe seats more competitive attempted to oust the commission's chairwoman. They failed, and the Justice Department approved the new maps in April, although lawsuits challenging them continue.