Midterm elections set to determine congressional control

The United States is heading into one of its most consequential midterm elections in decades. On November 3. Every seat in the House of Representatives and a third of the Senate will be up for grabs in what amounts to the country’s first major verdict on Donald Trump’s second term as president.

It was a year and a half ago that Trump pulled off a sweeping victory against Democrat Kamala Harris. His fellow Republicans also achieved narrow majorities in the Senate and House, giving Trump the power to push through legislation and cabinet appointments with minimal Democratic interference. With his approval numbers declining. Trump is pushing hard to retain Republican control of Congress.

Now with that power on the line, both parties are fighting not just for votes but also over the very maps where those votes will count. In this visual explainer, Al Jazeera unpacks what happens during the midterms, how the primaries work and how the shape of electoral district maps can favour one political party over another.

Structure of the midterm elections

On November 3, nearly 244 million voters are eligible to cast ballots in the US midterms. These elections are not to elect a new president but rather to vote for 35 US Senate seats and all 435 House seats. Voters will also elect 39 state governors and state legislators.

The Senate, or upper house of Congress, has 100 seats in total with each of the 50 states represented by two senators regardless of their population size. In the national elections held in November every two years, roughly one-third of the Senate’s seats are up for grabs as senators serve six-year terms and elections are staggered.

The House of Representatives, or lower house, allocates seats to each state according to population for a total of 435 members, all of whom serve two-year terms and are up for election every cycle. In 2024, the Republicans narrowly retained control of the House with 220 seats to the Democrats’ 215, the slimmest majority since 1930. That razor-thin margin is why the redistricting battle matters so much.

Redistricting and gerrymandering

Before any primary takes place, each state must draw its congressional district maps to determine which voters are grouped together and will vote for a single person to represent them in the House. These maps determine which voters are in each district, whether a district leans Democratic, Republican or is competitive, and how “safe” or “fragile” a seat is.

This process is called redistricting. Every 10 years after the US census, the country’s 435 House seats are reapportioned based on population shifts, triggering a nationwide redrawing of congressional districts. The most recent census was in 2020, and states completed their redistricting by 2022. Since 2024, however, several states have redrawn their congressional maps again – some successfully, some blocked by the courts – with accusations of gerrymandering.

Gerrymandering is the practice of drawing electoral district boundaries to favour one political party over another. The term itself originates in the US, coined in 1812 after Massachusetts Governor Elbridge Gerry signed a bill creating a strangely shaped district that a newspaper likened to a salamander, combining it with his name to get ‘gerrymander.’

There are two main forms of gerrymandering in the US: Packing and Cracking. Packing concentrates voters from one party or identity group into a small number of seats, limiting their influence elsewhere. Cracking splits those same voters across multiple districts, diluting their voting power and preventing them from forming a majority.

States redraw maps ahead of 2026 midterms

Before the 2026 midterms, at least eight states – California, Florida, Missouri, North Carolina, Ohio, Tennessee, Texas and Utah – passed new congressional maps, according to the National Conference of State Legislatures. On May 13, the National Association for the Advancement of Colored People (NAACP) along with a coalition of civil rights and community organisations filed a lawsuit In Tennessee against the lawmakers who split the state’s only majority-Black district centred on Memphis.

Legal battles over redistricting also escalated in Texas, Louisiana, Virginia, Alabama and South Carolina over racial gerrymandering and voting rights before the midterms. The map below shows redistricting activity between the 2024 and 2026 elections.

Florida’s new congressional map is expected to strengthen Republican control of the state’s 28 House seats and could help the party gain up to four additional Republican-leaning districts before the midterms. The previous map already favoured Republicans with Democratic voters concentrated around Orlando, Tampa and South Florida, including Miami. The redraw further clusters Democratic voters into fewer districts while expanding Republican-leaning areas across central and southern Florida.

Governor Ron DeSantis pushed the new district map into law. Voting rights groups and Democratic organisations filed lawsuits within days, arguing it violates Florida’s ‘Fair Districts’ antigerrymandering amendment and was designed to benefit Republicans. Texas remains a major redistricting battleground before the midterms. Republicans hold the majority of the state’s House seats at 25 to the Democrats’ 13.

On April 27, the US Supreme Court reinstated the post-2020 congressional map drawn by Texas Republicans, which was challenged under the Voting Rights Act in a lower court that had blocked the map over allegations of racial gerrymandering. Civil rights organisations – including the American Civil Liberties Union, the League of United Latin American Citizens and the Mexican American Legal Defense and Educational Fund – have challenged the map in federal court, arguing it violates the Voting Rights Act by diluting minority representation.

Missouri Republicans redrew congressional maps before 2026, aiming to gain one extra House seat for their party. In September, a new map was approved by Republicans to reinforce the party’s already strong advantage in the state’s US House representation. Heading into the redistricting effort, Republicans represented six of Missouri’s eight congressional districts.

Governor Mike Kehoe, a Republican who is closely aligned with the party’s conservative wing, signed the map into law in the same month for use in the 2026 elections. The new lines generally make most Republican-leaning districts in the rest of Missouri even harder for Democrats to win. The map faced legal challenges, but the Missouri Supreme Court upheld it, clearing the way for its use in the 2026 midterm elections.

In October, the North Carolina Senate approved a new congressional map that is expected to make one more US House seat Republican-leaning by