Should a potential so-called blue wave arrive, Mr Trump would find it exceedingly difficult to push through the kinds of legislation that he and his administration have so far been favouring during the first half of his presidency.
But, Mr Trump could also face a different hardship if Democrats regain control of the House: Impeachment.
Here is what you need to know about that possibility.
Will Democrats retake the House?
This is a good place to start.
While it is not clear that Democrats will actually take back the House, pundits and polls indicate that they have one of the best chances for doing so in years.
There are several good signs that Democrats, who need to pick up a net of 24 seats, will regain control of the people’s chamber.
First off, polls show that, nationwide, Democrats are favoured to be the party in power by the electorate 47.9 to 40.7, according to an average of the generic congressional vote compiled by Real Clear Politics. That ranking has no sway on individual races, but shows that Americans in general have more of an appetite for Democrats in 2018 than they do Republicans.
To get a bit more in the weeds, there are already about a dozen or so seats where Democrats are seen as having made enough gains on Republicans to flip the seat to their side.
In addition to that, more currently Republican districts themselves are considered to be either toss-ups or only leaning Republican this November compared to the same for Democratic seats — indicating that Republican seats are more vulnerable. Altogether, Cook Political Report finds that 53 GOP-held seats are in play, compared to just four for the Democrats.
If they manage to take over the House, then they would have the numbers to impeach Mr Trump.
What is impeachment, anyway?
Impeachment does not guarantee removal from office, but it is a serious situation for the president (or other high government official) to find themselves in, and it is the first step toward removal from office.
Technically speaking, impeachment is a formal charge of misconduct levied against a public official by Congress. It is not in any way a verdict or finding of criminality in and of itself. A majority of the House can vote to impeach the president.
How does impeachment work?
In the United States, the House of Representatives has the sole right to initiate impeachment charges against a high official in government.
The process begins with an independent investigation — this could come from the Department of Justice, from Congress itself, or from an appointed special council — which findings for the basis of charges. That evidence is then handed over to the House Judiciary Committee, which reviews evidence and writes up the Articles of Impeachment.
After debate in the House, the legislative body will vote on the Articles of Impeachment. If a simple majority votes in favour of impeachment then the president is considered impeached.
What would happen next?
From there, the Articles of Impeachment are sent over to the Senate for consideration and a trial. The accused mounts their defence and the House Judiciary Committee acts as the prosecution. During the proceedings, the Chief Justice of the Supreme Court acts as the judge and the Senate acts as the jury.
If a two-thirds majority in the Senate votes against the President, that’s it. The President is removed from office.
Would Democrats actually pursue that route?
This is not clear, and it may be unlikely, especially if they do not also gain a miraculous number of seats in the Senate.
Some Democrats have suggested they would like to impeach Mr Trump if they regain the House — but doing so would still be a largely symbolic move at this point.
The likely speaker of the House if Democrats regain control, Nancy Pelosi, has frequently sought to advise her party against talk of impeachment. While that may be practical — the president is likely to stay in office either way — Ms Pelosi may be making a judgement call that impeaching Mr Trump could motivate his voting base during upcoming elections.