It's a significant political event that happens once every 10 years: the Census announces its population count for each state.
Here’s a guide to the politics that will follow from that Census count, due out Tuesday.
What will the Census reveal on Tuesday?
The Census will announce each state’s population, based on this year’s tally, and the number of representatives that each state will be allocated in the House of Representatives.
The number of seats in the House was fixed by Congress in 1911 (and reaffirmed in 1929) at 435.
The Constitution apportions House members according to population, so Texas, for example, gets 32 seats while West Virginia gets only three.
Every 10 years, after the Census Bureau tallies the population, some states gain seats, others lose them, to reflect the faster population growth in some states than in others. This process of reallocating seats is called reapportionment.
Reapportionment measures long-term shifts in political power.
New York, for example, went from having 45 House members in the 1940s to 29 today. At the same time, Florida’s representation grew from six members in the 1940s to 25 today, a quadrupling of its clout in the House.
To see how states’ apportionment of House members has changed since the first Congress, see this table.
The Constitution specifies that "each state shall have at least one representative," which means that states such as Wyoming will always have one member no matter how small their populations remain.
Which states are likely to lose seats as a result of the 2010 count?
According to an estimate done in October by the demographic firm Election Data Services, states either certain or likely to lose seats in the House include Illinois, New York, New Jersey, Ohio, Michigan, Missouri, Pennsylvania, Louisiana, Massachusetts and Iowa.
Which states are likely to gain seats?
According to Election Data Services, Texas, Arizona, Nevada, Utah, South Carolina, Florida, Georgia and Washington would gain seats, with Texas picking up as many as four.
Is it mandatory for a state to have lost population since the 2000 Census in order to lose seats in the House?
No. If a state’s population is growing slowly relative to other states’ populations, it could lose seats in the House.
What’s the ratio between the number of people in each state and the number of representatives it has?
Based on the apportionment after the 2000 Census, each member of the House represented an average population of about 647,000. But four states — Vermont, Wyoming, Alaska and North Dakota — had populations than were smaller than 647,000.
Since the Constitution specifies that each state shall have at least one representative, each of those states got one even though their entire state populations were smaller than the populations of congressional districts in other states.
Given the nation’s population growth since 2000, the new ratio is likely to be on the order of one member for every 710,000 people.
How long does it take for the reapportionment of seats to occur?
Reapportionment takes effect two years after the last Census. The apportionment after the 2000 Census took effect in time for the 2002 elections, which elected the Congress that convened in January 2003.
If a state has many illegal immigrants living in it, will it get more representatives in the House?
Yes. All people, citizens and non-citizens, legal residents and illegal residents, are included in the count used to determine state population and representation. But illegal immigrants may have shunned contact with Census enumerators and therefore may be undercounted.
Does reapportionment have an effect on the 2012 presidential election?
Yes, it could have a significant effect on the 2012 presidential election, if it’s a close one.
Presidents are elected by the Electoral College, not by direct vote of the people. Each state gets a number of presidential electors equal to the number of its members of the House plus two. In the 2008 election, Texas, for example, had 34 electoral votes. As a result of reapportionment, Texas may have as many as 38 electoral votes in the 2012 election.
Notice that of the eight states cited above as likely to gain representation — and thus gain electoral votes — President Barack Obama won only three of them: Nevada, Washington and Florida.
Of the states that are likely to lose electoral votes, Obama won all of them except for Louisiana and Missouri.
Once reapportionment has determined how many seats each state will have, how are the district lines drawn within each state?
In 43 states, the state legislature draws the new lines, a process called redistricting. In all but one of those states, the governor has veto power over the plan drawn by the legislature.
Legislatures often design districts to give one party an advantage, a process called gerrymandering.
In a few states a nonpartisan or bipartisan commission does the redistricting.
Also some states come under the restrictions of the federal Voting Rights Act and thus their redistricting plans will need to be submitted to the Justice Department for approval. This often has ended up in court battles.
How did last November’s elections affect redistricting?
Republicans made massive gains in state elections which will help the GOP protect its new majority in the House by drawing lines favorable to Republicans.
According to Tim Storey of the National Conference of State Legislatures, of the 435 House districts, 195 are in states where GOP legislatures and Republican governors will draw the lines; only 49 of the House districts are in states where Democrats will entirely control the line drawing.
Republican strategist Ed Gillespie, who helped direct GOP redistricting effort, said GOP legislators will be in position to redraw the maps to protect between 15 and 25 Republican seats.
“I’m not sure how much we’re going to gain (from redistricting) in terms of seats because it was such a big election for (House) Republicans,” Gillespie said after the election. “I think we’re going to end up protecting a lot as opposed to carving new ones.”
Isn’t partisan design of congressional districts a politicizing of the process?
Yes, it is. “This is the most purely political process in politics,” said a leading Democratic expert on redistricting, Tom Bonier of the National Committee for an Effective Congress.
In a 2004 decision, Vieth v. Jubelirer, the Supreme Court said there was no legal standard to figure out whether a redistricting plan was so partisan that it violated the Constitution. Justice Antonin Scalia wrote that the “unanswerable question” for the court was: “How much political motivation and effect is too much?”
What states are worth watching as the redistricting process unfolds over the next several months?
Bonier said the redistricting struggle in New York “is incredibly important.”
In the November elections, Republicans won five House seats in New York that had been held by Democrats. Since the GOP also won control of the New York State Senate, “we’ll have a better chance of protecting those five freshman Republicans so that they’re not drawn out of their seats — especially in a state that is going to lose two seats in the apportionment process,” Gillespie said.
Bonier also notes that of the states projected to lose at least one congressional district, Democrats will control the entire redistricting process in two (Illinois and Michigan). Republicans will control the process in three of them (Michigan, Pennsylvania and Ohio), the control is split in three (Louisiana, Missouri and New York) and two states will have the maps drawn by commissions (Iowa and New Jersey).
“Illinois stands out as the most important state likely to lose a district that Democrats will control,” he said.
“Democrats won't control the entire redistricting process in any of the eight states projected to gain a district, while Republicans will have complete control in five of those states.”
But in Florida a constitutional amendment bans partisan gerrymanders. That, Bonier said, “will make it difficult for the Republicans to preserve what is the most gerrymandered map among all fifty states. The fact that Florida could gain two seats in reapportionment and Democrats currently control only six of 25 congressional districts (in the new Congress) makes the outcome of redistricting here even more critical.”