Redistricting in the states occurs every 10 years after the census, but loose laws and backroom deals have formed "safe districts". Safe districts are fragmented in such a way as to guarantee that it will vote more Republican or more Democrat. This process is called "gerrymandering".
Some of the most gerrymandered districts can be found at http://t.co/gsdvxNIS
Creating new standards of redistricting, and ensuring that those standards are fair, will lead to more competitive elections, more moderate districts, and, overall, a less polar and feckless Congress.