On 3 November, halfway through Donald Trump’s second term, Americans will go to the polls in the midterm elections. All 435 seats in the House of Representatives, the lower chamber of Congress, are being contested, along with 35 of the 100 seats in the Senate, the upper chamber.

House seats are allocated among the states according to population, so California, the most populous state, has the largest number, with 52. In the Senate, every state has two seats, regardless of its size, and senators are elected directly by the people.

Midterm elections are widely seen as a report card on the President, and this one looks unfavourable for Trump. A Reuters/Ipsos poll this week put his approval rating at 31 per cent, the lowest of his political career. When asked which party they wanted to control the US Congress, voters in the same poll favoured the Democrats by 12 percentage points.

The ruling Republicans have a slim majority in the House, so the Democrats need only a...