The mid-terms run every four years and are when Americans decide their next members of Congress, the law-making government body.
The elections get their name because they run in the middle of the US President’s four-year term office.
Congress is made up the House of Representatives, the lower chamber, and the Senate, the upper chamber.
Voter turnout for mid-term elections are lower in general than presidential elections but a recent survey by the Pew Research Centre found voter enthusiasm for the upcoming mid-terms are at their highest in 20 years.
When are the US mid-terms? Why are they so important for Donald Trump?
The 2018 US mid-terms are on November 6.
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They are significant for Donald Trump because the results could shape the US President’s ability to pass legislation in the future.
At the moment Mr Trump’s Republican Party controls power in both houses.
But all 435 seats in the House of Representatives are up for grabs – and the Democrats are confident they can swing the result in their favour.
Recent opinion polls have tipped the Democrats to win the majority of seats.
Forecasting website FiveThirtyEight’s most recent statistical analysis on October 10 found the party has an 84.4 percent chance of controlling the house.
In the senate, only 35 out of 100 seats are up for grabs and a Republican majority is looking more likely.
But if each party controls one half of Congress, this could see new legislation becoming blocked on a regular basis.
Mr Trump could see his plans to build a wall between the United States and Mexico stumped.
But the real concern for the US President will be Democrats’ power to trigger articles of impeachment against him if they win the House.
If that happens, Mr Trump could be put on trial for charges of “treason, bribery, or other high crimes and misdemeanours” by the Senate.
No US President has been removed from office by impeachment but two previous presidential impeachment trials have been held.
The first was against President Andrew Johnson in 1868 for firing a cabinet secretary without Congress consent.
President Bill Clinton was also tried for perjury and obstruction of justice over a sexual harassment lawsuit brought against him by Paula Jones.
Both men were acquitted and stayed in office.
President Nixon would have faced impeachment charges over the Watergate scandal but resigned before they could be effected.