President Barack Obama’s Democratic allies and Republican foes are battling over a once-in-a-decade prize in November mid-term elections: power to literally redraw the US political map.

Each US state gets two senators, but the 435 full members of the House of Representatives are divided among the 50 states depending on their population, meaning some will gain or lose seats based on the results of the 2010 US census.

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The November 2 elections will decide which party controls key state legislatures and governorships that typically carry out the task of redrawing a state’s congressional districts, a job that can be bent for a partisan edge.

“The redistricting process has a large impact on whether or not any given seat is contestable,” said Justin Buchler, an expert on redistricting who teaches political science at Case Western Reserve University in Ohio.

“A Democrat in a district where 40 percent of the voters are Democrats and 60 percent are Republicans, the Democratic candidate will not have much of a chance,” Buchler told AFP by telephone.

So politicians have sometimes sought to pack their opponents in as few constituencies as possible, or divided them so that they are in the minority in several districts, tactics to reduce their presence in the US Congress.

In the best-known example, then Massachusetts governor Elbridge Gerry signed legislation in 1812 to redraw his state’s districts to help his Democratic-Republican party, including one constituency that a newspaper decried as resembling a salamander — and the term “gerrymander” was born.

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The behind-the-scenes battle has already begun, with lawyers on both sides preparing for a fight that commonly spills into the courts, which have increasingly decided redistricting battles over the past few decades.

The formal two-phase process begins at the end of December when the US Census Bureau reports to Obama on the population figures for all 50 states, as well as the number of seats each state will have going forward in the House — a step called “apportionment”.

Detailed population figures are formally reported shortly thereafter to the states so that they can begin the next phase, “redistricting”, the actual act of redrawing the map.

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In most cases, that job will fall to state legislatures, often with the governor playing a key role, though some states rely on independent commissions and the smallest have only one House member.

The US Census Bureau has yet to formally determine which states will gain or lose House seats based on the latest congressionally mandated 10-year population count, completed April 1, said spokesman Derick Moore.

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But independent observers have forecast that Republican-leaning Texas could gain up to four House seats through apportionment, and Florida may get one more, while traditional battleground Ohio could lose two.

The resulting shuffle will slightly alter the balance of US presidential races, in which each state has as many votes in the electoral college that decides such contests as it has representatives and senators in congress.

Another historic, though controversial, use of redistricting has been to try to ensure that racial minorities can win office in the winner-take-all US system.