How the shape of a map can determine the US midterm elections

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.

(Al Jazeera)

What happens during the midterms?

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.