American voters will select 35 out of 100 Senators, as well as all 435 members of the House of Representatives.
These Congress elections – the Senate and the House make up Congress – are crucial for Donald Trump.
If the Democrats take back control of the house – which is often how midterms tend to swing – it could severely limit what Mr Trump can do for the final two years of his tenure.
But after a bitter fight over the confirmation of Brett Kavanaugh to the Supreme Court, what was once seen as an opportunity for the Democrats to make inroads on the map, is now looking more contentious.
WHEN ARE THE MIDTERMS?
Voters will head to the polls on Tuesday, November 6.
HOW DOES IT WORK?
House:
In the battle for the House, which is the lower chamber of Congress, all 435 seats are up for election – 218 seats are needed for control.
Currently, the Republicans hold 240 seats, and the Democrats hold 196.
Democrats will need to gain 24 seats to flip the house, but voter turnout will be key.
Senate:
Only 35 out of a total 100 seats are up for election in November.
These are staggered, as members of the Senate serve six-year terms.
WHAT DO THE LATEST POLLS SHOW?
What was once looking set to be a good chance for the Democrats to gain some ground has taken a knock since the Kavanaugh scandal.
Republicans who were less inclined to vote have ben rallied, and seats in danger zones are looking more fragile.
But the Democrats do still have a slight advantage in the polls.
A Wall Street Journal/NBC poll shows 50 percent of likely voters prefer Democrats, while 41 percent prefer Republican control.
Among all registered voters, a broader group of respondents, Democrats’ advantage over the Republicans is narrower — 48 to 41 percent.
Democrat Fred Yang, who conducted one opinion poll, said the widely predicted “blue wave” of Democratic gains in the House in 2018 now is running into a “riptide of uncertainty that has been created with a surge of Republican intensity.”
Although Democrats are preferred in the national poll overall, their advantage has vanished in the House districts that matter most.
Of the 13 most competitive races, 10 are in states which voted overwhelmingly for President Trump.
Democratic senators like Heidi Heitkamp of North Dakota, Claire McCaskill of Missouri and Joe Donnelly of Indiana — who are all running in states Mr Trump won by double digits — have come under attack for their opposition to Justice Kavanaugh.
Democrats are hoping to capture four Republican-held seats. But in two of them — Texas and Tennessee — polls taken after the Kavanaugh confirmation showed their candidates slipping.
Democrats appear to be in closer races in Arizona, an open seat, and Nevada, where Senator Dean Heller is the only Republican incumbent up for re-election in a state Mr Trump lost.
Even if Democrats win both those states, they still must successfully defend all of their incumbents to take back the majority, and with just over two weeks until voters go to the polls, that is quite a challenge.