Lawmakers in California and Texas – the two most populous US states that together contain more than 70 million Americans – are at the centre of an intense political battle that is drawing in other state governments and is poised to have a major impact on the balance of power in Washington DC.

After Texas passed a redistricting measure that would create five more congressional seats favouring Republicans, California lawmakers hit back on by voting to re-draw the state’s maps – carefully made to cancel out Texas’ move.

Missouri then got in on the action, with the governor ordering a special session to begin on Wednesday so lawmakers could work towards redrawing a new map designed to add one more Republican seat.

This political arms race for power may seem both bizarre and confusing – but it is one that is rapidly spreading to other states in advance of next year’s national midterm elections.

Here’s what you should know about America’s fight over congressional redistricting.

The battle began in Texas this summer, when the Republican-majority legislature took the unusual step of re-drawing congressional seats in the middle of the decade – in a deliberate effort to send more Republicans to the House of Representatives in Washington.

The Lone Star State’s goal was to add five additional congressional seats, which would favour Republicans. California responded by re-drawing its own districts to boost Democratic representation by five seats, in an attempt to cancel out the move by Texas.

Missouri is the third state to join the fight, which comes after President Donald Trump called on loyal statehouses across the country to carve up their districts in order to maintain full Republican control of Congress after the November election.

The US House of Representatives is made up of 435 legislators who are elected every two years.

They represent districts with boundaries determined in processes set by their state governments. Some states have non-partisan independent commissions which determine districts, while others leave it up to the state legislature.

Who draws the lines and how can go a long way in shaping the ideological tilt of the district and the likelihood that it elects a Democrat or a Republican.

At the moment, the House rests on a knife’s edge, with Democrats only needing to seize three more seats to flip the chamber in their balance.

The president’s party historically loses seats in the midterm election after their victory.

If Democrats seize the House, they can launch sweeping investigations of presidential actions, as Democrats did in the second half of Trump’s first term and Republicans did in Joe Biden’s final two years.

Each member of the US House represents a district in their home state. The districts are typically set after the US Census is conducted every decade to account for the states’ population changes.

But Republicans and Democrats are now fighting to redraw the maps mid-decade, either to help or block Trump’s agenda. By changing the make-ups of the districts, each party hopes to capture more seats in the House.

Trump has called on Republican-controlled statehouses across the country to re-draw their districts to try to prevent his party from losing control of the House in the November midterm elections.

But the effort in Texas hit a roadblock in early August after the state’s Democrats fled the state in a bid to deny the quorum needed to pass the Republican redistricting measure.

They returned after two weeks, when the governor ordered them to be arrested. After they returned, the chamber voted 88-52 to create five new seats.

The measure was swiftly endorsed by the Senate and then signed into law by Governor Greg Abbott.

Democrats and civil rights groups have said Texas’ new maps will dilute voting power from minorities, in violation of the federal Voting Rights Act, and have threatened to sue.

Gerrymandering – the redrawing of electoral boundaries to favour a political party – is practised by both main parties and is legal unless ruled to be racially motivated.

However, it is extremely unusual for the president to publicly back a state’s effort to create partisan advantage. While both Democratic and Republican-dominated states have faced criticism and court battles over their US House maps, rarely do lawmakers so explicitly acknowledge the partisan intention behind their actions.

Critics argue that the process allows politicians to pick their voters, rather than voters picking their elected officials.

It can lead to some very wonky-shaped maps, in which certain groups are lumped together regardless of their geographic proximity.

In 2019, the Supreme Court took away the power of the courts to block political gerrymandering.

“Federal judges have no license to reallocate political power between the two major political parties,” Chief Justice John Roberts wrote in his ruling at the time.

Redistricting could have significant implications in the 2026 midterms, and has become an urgent issue for state lawmakers.

Illinois, New York, New Jersey, New Hampshire and Maryland are among the Democratic-led states prepared to launch countermeasures to the Texas plan.

Indiana, Florida, and Ohio are also now looking into redistricting in order to boost Republican representation in Washington.

Missouri will hold a special session staring on 3 September to redraw maps, which target a Kansas City-area district held by a Democratic representative.

Missouri Governor Mike Kehoe said the new map will “ensure our districts and Constitution truly put Missouri values first”.

California Governor Gavin Newsom, whose state is leading the Democratic charge, has pledged to “fight fire with fire” by replicating trying to boost his party’s chances of victory.

But the process in California is slow, due to a 2008 law designed to make the process less partisan.

On 21 August, the California statehouse voted to have the new maps put on the special election ballot in November, to let voters decide if they should shift.

The vote may prove controversial, as voters had previously approved a process that leaves it up to a non-partisan commission to decide how voting lines should be drawn.

Utah, with four districts all held by Republicans, has been ordered by a judge to redraw its maps ahead of the midterms.

The Utah judge ruled that lawmakers had disregarded a rule passed by voters that bans politicians “from favoring or disfavoring incumbent elected officials or from considering partisan political information” when drawing their maps.

The move is expected to end up favouring Democrats in Salt Lake City, the capitol of the Beehive State.

New York wants to enter the fray, but there the process takes even longer, as the state constitution would need to be changed.

“In New York, we’ll confront Trump’s legal insurrection head-on,” New York Governor Kathy Hochul pledged.

“We’ll meet him on the same field and beat him at his own game.”

According to experts, states where Republicans control most of government may have an easier time redrawing their maps.

Democratic-controlled states have more legal and constitutional barriers designed to protect against political gerrymandering.

Several smaller states, including Delaware and Vermont, only get one seat in the House – making them largely irrelevant to the current fight.

But with so much at stake, even these smaller states may get in on the action eventually.