Two years after Joe Biden was elected US president, voters return to the polls on 8 November.

Congress is made up of two chambers - the Senate and the House of Representatives.

The Senate is the 100-strong upper chamber.

The Democrats control both chambers, but by very slim majorities.

So far, this has made it easier for President Biden, a Democrat, to get things done.

But if the Republican Party gains control of either or both chambers, it will have the power to thwart the president's plans.

At present, the Democrats have control because Vice President Kamala Harris has the casting vote in any tie.

The Republicans only need to win one extra seat to gain control in November.

Primary elections to determine who will contest the general election for each party will take place around the country between May and September.

Historically, the party that holds the White House has tended to suffer losses in the midterms.

Conservative Democrats - not Republicans - have blocked several major proposals, including "Build Back Better", the president's trillion-dollar package for social programmes and climate action.

As well as the elections for Congress on 8 November, 36 out of 50 state governors are up for election.