The most crucial race in these elections is for Congress, the branch of government which makes US law.
Congress is made up of two chambers, the Senate and the House (the House of Representatives).
At the moment both chambers are controlled by Donald Trump’s Republican party, but the Democrats have fought hard to make inroads, and polls are showing a neck and neck race.
Midterm elections are widely viewed as a referendum on the sitting president, something Donald Trump has enforced with a ferocious spell of campaigning in the run-up to the November 6 vote.
Who will win?
Recent polling shows the Democrats have a good chance of taking control of the House.
All 435 seats in the House are up for grabs, and a party needs 218 to be in control.
If the Democrats want to flip the House of Representatives, they will need to take at least 23 seats from the Republicans.
READ MORE: Follow our LIVE UPDATES as America heads to the polls
Polling site FiveThirtyEight gives the Democrats an 87.3 percent chance of this manifesting.
The Senate, however, paints quite a different picture.
In the Senate, only 35 out of the total 100 are up for grabs, as the duration of sitting senators is staggered.
This year, Democrats have more seats up for re-election (24) than the Republicans (nine), and the majority of these seats are in states that overwhelmingly in favour of Mr Trump in 2016.
In fact, Mr Trump won 10 out of 13 states with close Senate races.
Polling site FiveThirtyEight gives the Republicans an 83.3 percent chance of retaining Senate control.
READ MORE: When will results come in?
Why is this so important to Trump?
The result of this election will determine how strong Mr Trump’s power will be for the rest of his tenure.
Proposed legislation must pass in the House and the Senate for it to reach the president’s desk, and whichever party controls the chamber usually wins the legislative votes.
If either chamber falls to the Democrats, the ability of Mr Trump to make and pass new laws would be notably restricted.
But if the Republicans hold both chambers, it would serve as an endorsement of the Trump administration, and allow him to ramp up on controversial proposals such as the Mexico border wall and abortion law.
READ MORE: What do the midterms decide?
What do the bookies say?
According to bookmakers Coral, the Democrats are 4-9 odds on favourites to win the House.
Odds show the Republicans only have a 13-8 chance of a majority.
But despite the polls, the odds are not favourite Republicans in the Senate, with bookies betting on a blue wave.
Current odds are at 2-9 for the Republicans to take a majority of more than 50 seats.
There is a 6-1 chance the Republicans will take exactly 50 seats and 7-1 odds they will have under 50 seats.