US midterms explainer: The lowdown on the 'Trump referendum'
But what exactly are the midterms, and why do they matter so much this time round? Here's °®Âþµº's guide to what these crucial elections mean for the US, as well as what to look out for and when.
What are the midterms?
The US midterms are elections held across the country every four years and fall halfway through the sitting president's four-year term in office. Voters will cast ballots for a number of elected positions, however seats in Congress, which is made up of the House of Representatives and the Senate, will take the main stage.
As the House holds elections every two years, all 435 seats will be voted on. Meanwhile, one third of Senate seats - 35 - are up for grabs on Tuesday, as elected representatives in the upper chamber serve staggered six-year terms.
The Republicans currently have a majority in both chambers of Congress.
- 435 House seats up for grabs
- One third (35) Senate seats
- Governers' elections in 36 states
- Smaller local elections across the country
Local and lower level elections will also take place, with 36 states running elections for governors.
'Referendum' on the president
The midterms are also seen as an opportunity for voters to cast a verdict on the incumbent president, with voters able to show their approval or otherwise of the leader by voting for candidates from the same party - or the opposition.
"In a sense, I am on the ticket," President Donald Trump told attendees at a Republican Party rally in Ohio.
With Americans increasingly divided over issues ranging from firearms laws to abortion rights under the Trump administration, the vote will play a crucial role in setting the tone for the rest of President Donald Trump's four-year term.
Most diverse elections in US history
President Trump's anti-migrant and at times xenophobic sentiment which has emboldened America's far-right is contrasted by the fact that Tuesday's elections will feature the most diverse pool of candidates in US history.
Across the US, a record number of women, LGBTQ and candidates from a range of ethnicities will contest positions.
The US may soon see its first female Muslim representatives in Congress with Rashida Tlaib running in Michigan and Ilhan Omar standing for election in Minnesota.
Tlaib, a Palestinian-American lawyer, is facing a last minute write-in challenge from fellow Democrat Brenda Jones.
Tlaib defeated Jones in a Democratic Party primary in August by 900 votes.
Omar, 37, is hoping to replace Rep. Keith Ellison, the first Muslim man elected to Congress. She is the assumed favourite to win the seat for Minnesota's 5th congressional district.
Why do the midterms matter?
If the Democrats are able to steal a Senate majority from the Republicans, the opposition party will be able to block supreme court and cabinet appointments.
A majority would also allow the Democrats to stifle the Trump administration's ability to carry out its policy programme. In theory, impeachment proceedings against the president could also be initiated with a House majority, however the Democrats lack the two-thirds Senate majority required to remove the president.
What time?
Polling began at 8am EST (1pm GMT) and the first votes closed at 7pm EST (midnight GMT). By 3am EST (8am GMT) on Wednesday, observers will have a clearer picture of how the vote went and what the implications are. Exit polls, however, are likely to be announced as the first votes close.