Voting in New Jersey, Virginia, Connecticut and Kentucky will start at 6am (11:00 GMT). All 435 House of Representatives seats and 35 of the Senate’s 100 seats are up for grabs.
Voters in 36 states will also elect governors.
US midterm elections live: Polls due to open in test for Trump
The first results are expected on Tuesday, as polls begin to close at 23:00 GMT, but final results might take days.
