US voters have gone to the polls to determine who controls Congress, as well as picking new mayors, governors and state legislatures.
The federal results may define the rest of Donald Trump's presidency.
Here's why it matters.
What are the midterms?
Midterm elections occur halfway through a president's four-year term and are considered a check on their performance.
While the president is not on the ballot, Americans will decide who controls the two chambers of US Congress - and whoever controls these chambers sets the legislative agenda for the world's most powerful country.
All 435 seats in the House of Representatives (the lower chamber) are in contention, while around a third of the 100-member Senate (the upper chamber) is up for grabs.
Voter casts his ballot in the 2018 mid-term general election at First Presbyterian Church in Ferguson, Missouri USA Source: EPA
People will go to the polls on Tuesday, although nearly all 50 US states allow some form of early voting.
Republicans currently hold both chambers - with a 235-193 advantage in the House and a 51-49 edge in the Senate. Democrats would need to gain an additional 23 seats to reclaim the House.
A two-seat Senate gain would give Democrats control in that chamber, but this year's Senate electoral map is particularly challenging, as Democrats are defending 26 seats compared to just nine for Republicans.
US President Donald Trump speaks to supporters during a rally in Cleveland. Source: EPA
What's at stake?
The impact of the 2018 midterms could be enormous, beyond just whether Congress will support or impede Mr Trump's agenda.
Should Democrats flip the House, the likelihood of impeachment proceedings against Mr Trump would increase dramatically.
Investigations into Mr Trump's administration, including the probe about his campaign possibly colluding with Russia, would also intensify.
Powerful committee chairmanships shifting to the Democrats could result in a new round of subpoenas.
And Democratic control of the Senate, which votes on the president's nominees, would make it harder for Mr Trump to get any new picks onto the Supreme Court should a vacancy occur.
How are the parties campaigning?
Dr Charles Edel, senior fellow at the United States Studies Centre, told SBS News the midterms are "largely a referendum on President Trump".
"By all metrics – a lot of people are saying we're looking at whether the United States wants checks on Donald Trump or if they want to keep things as is," he said.
As a result, many Democrats are putting Mr Trump at the centre of their campaigning.
In conservative states including Kansas or South Carolina, there is little reason for Republican candidates to distance themselves from Mr Trump, as he remains relatively popular there despite high overall disapproval numbers.
In battleground congressional districts, Republicans are focusing more on strong economic growth than on Mr Trump, while Democrats are highlighting his controversial immigration, health care and trade policies.
Who will win?
Looking back at the past 21 midterms, the president's party has lost an average of 30 seats in the House of Representatives and an average of four seats in the Senate.
There have only been two occasions where the president's party gained seats in both houses.
"And that's generally because American people like divided government," Dr Edel said.
"Enthusiasm is always at its peak right after someone's been elected and then they have to govern, and so traditionally ... the president's party ends up losing seats."
What are the races to watch?
Texas is hosting the most closely watched - and most expensive - Senate race this year. Beto O'Rourke, a punk rocker-turned-congressman who espouses health care for all, criminal justice reform and stronger gun-safety laws is taking on popular conservative Senator Ted Cruz.
Flipping deep red Texas has been a Democratic dream for years. They will need suburban anti-Trump anger to help them pull off such a political earthquake.
Alexandria Ocasio-Cortez made headlines earlier this year when she toppled Democratic Party grandee Joe Crowley in a primary vote.
Should she win in the midterms, the 29-year-old self-proclaimed "democratic socialist" would be the youngest woman ever elected to the House.
The election is also shaping up to be a big year for LGBTQI+ candidates.
Christine Hallquist, the Democratic nominee for governor of Vermont, could become the first ever transgender governor of a US state.
Over in North Carolina's state legislature, Peter Boykin is the founder of the organisation Gays for Trump. Unlike most of the other LGBTQI+ candidates running this year, he's a Republican.
In several races there are candidates vying to become the first Muslim woman in Congress, 12 years after Minnesota's Keith Ellison became the first Muslim in the House of Representatives.
And three states, Florida, Georgia and Maryland, could also see their first black governor. The US currently has no black governors and has only ever elected two in its entire history.
