It happens once every 10 years. Yesterday the US census came out and with it ostensibly good news for the Republicans in 2012. As the census displays growth both nationally as well as state by state, this means a reapportionment of seats in the House of Representatives. States whose population growth surged will gain seats; the laggards lose seats.
This does not affect the Senate where every state, regardless of population size, elects two senators. However, it does shift matters in the Electoral College, since a state's congressional delegation (House members plus two senators) represents its total of electors. The states that Obama won in 2008 have lost together 6 electors and the states taken by his rival John McCain have picked up six on aggregate. If the election is on the order of 2008 -a landslide - then the net change of 12 seats will not matter, but if it reverts to the closely fought 2000 and 2004 contests, the change could prove very meaningful.
The big gainers were Texas and Florida that picked up 4 and 2 seats respectively, while the big losers were New York and Ohio that will see their house delegation pared by two seats apiece. The Democrats sought solace in the fact that the states gaining seats have witnessed an influx of Hispanic voters who have been trending towards the Democrats primarily due to that party's more tolerant attitude towards immigration.
18 states in toto either gained or lost seats and this will result in drawing new congressional districts. The ways that the new districts are drawn may determine the fate of incumbent congressmen in the states that have lost seats. Obviously each party would like to draw districts to its advantage and here the Republicans derive another benefit from their success in the 2010 elections. The state legislatures are responsible for redrawing the districts and the Republicans captured quite a few state legislatures in the recent elections, so they command an advantage.
The overall gain by the southern and western states with their more conservative traditions can also somewhat push legislation more in the direction of limited federal intervention and accentuate a divide between a more liberal Senate and conservative House of Representatives.