Congress is made up of two parts – the lower chamber, the House of Representatives and the upper chamber, the Senate.
To win a majority in the House, a party needs 218 seats. In the Senate, a party needs 51 seats for an outright majority. But if the upper chamber is split 50-50, the vice-president has a tie-breaking vote, which gives their party control of it.
First results will start to come in after 18:00 EST (23:00 GMT). They won’t tell us much about the overall result because these races are in places considered safe bets for Republicans (Kentucky and Indiana) or Democrats (Vermont).
Throughout the night, BBC analysts will focus on the competitive races which will give an indication of how well things are going for Trump.
There are several tight Senate contests in the Eastern Time zone – like Ohio, where polls close at 19:30 EST (00:30 GMT) and Maine, where they shut at 20:00 EST (01:00 GMT). These could provide early indications of the electorate’s mood.
Some key contests might start to be “called” by US media outlets. They use models to project which way a state has voted, even before the full vote count has been completed.
This happens when they believe a party or a candidate has gained a lead that cannot be beaten. But in some closely-fought states, outlets could take a long time to make their projections.
The overall timing of the result will depend on how tight the races are and which races end up being decisive.
