One third of the Senate and the whole House of Representatives is up for grabs, as President Joe Biden faces his biggest electoral test since the 2020 presidential vote.
Results will start coming in after 11pm GMT, our live maps and charts will update throughout the night showing the progress of both races.
The race for the House
What is the House of Representatives and how are seats allocated?
The race for the Senate
Including those seats which weren’t up for election this year, this is how the balance of the Senate will look until the 2024 election.
Why aren’t all the states having elections?
What does this election mean for America?
After the 2020 election it was the first time since Barack Obama’s first term that both chambers of Congress and the presidency had been in Democratic hands.
It is much easier for a president or a party to enact their policies if all three are held by the same party.
If Joe Biden’s Democrats lose control of one or both chambers they will lose the ability to set the agenda on the big issues dividing the country, things like gun control, the economy, abortion, immigration, and climate change.
Governing parties do typically lose seats at the midterms, although the Democrats will of course be hoping to buck that trend as George W Bush last did in 2002, a year on from 9/11.
With President Biden’s approval ratings at a historic low however, doing that looks unlikely.
The question that remains is whether it will be enough for the Republicans to gain the five seats they need to take the House and the one they need for the Senate.
This page will be kept updated through the night with live results and more analysis.