"If you look back through the last 50 or 60 years of USA history, if you are an incumbent President, the chances are you will be re-elected and there is a strong pattern of the United States two terms for one party, two terms for the other".
And they appear particularly motivated to vote.
Whatever the outcome, Trump made clear he knew his political future was on the line. Here is a dummies guide.
Analysts at Nomura point out that tonight we will begin to get the U.S. midterm elections real results after polls start to close at 6pm EST and there will be lots of media coverage and the challenge will be making sense of the results.
Midterm elections are often the "lounge act" to the presidential contest, as she puts it, but the battle for the House quickly spun into the signature contest of 2018.
Midterm elections are not the same as state and local elections, which are held yearly.
Turnout is expected to be high, with more than 34 million Americans already having voted in the election.
All 435 seats in the House, 35 seats in the 100-member Senate, and 36 governorships are up for grabs as Americans cast their ballots.
On top of that, there are races for the Governorship of 36 states, plus a bunch of smaller races that could re-shape U.S. politics for at least the next few years. Voters will also decide on ballot measures such as former felons' right to vote.
The Senate is composed of senators, each of whom represents a single state in its entirety, with each state being equally represented by two senators. Candidates and their supporters are using the downtime to phone bank voters who still haven't voted and walk as many neighborhoods as possible.
If the GOP clings to the House and keeps the Senate, Trump would likely claim validation for his hardline leadership rooted in keeping his political base intact and vilifying opponents.More news: James Corden And Barbra Streisand Do Carpool Karaoke
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Democrats have accused Republicans of racism, corruption and financial profligacy. Almost 200 Democratic women are running for Congress this year, compared to just 59 Republicans. The party will therefore need to keep all 26 their seats and gain two seats to take control of the Senate - a tall order.
With a House majority, Democrats will have the power to investigate Trump's tax returns and possible conflicts of interest, and challenge his overtures to Saudi Arabia, Russia and North Korea.
A total of 155 Republican incumbents are defending House seats in districts Trump won by double digits in 2016.
"The vast majority of women voters are angry, frustrated and they are really done with seeing where the Republican Party is taking them, particularly as it related to heath care and civility", said Stephanie Schriock, who leads EMILY's List, a group that help elect Democratic women.
In the 1874 midterm, Republicans lost an astonishing 96 seats in the House of Representatives as Democrats took 168 seats, Republicans held 108, and independents held 14. All representatives are directly elected.
The Democratic party is favored to win the House, where a wave of Republican retirements created open seats in competitive districts throughout the country.
Democrats are far better positioned for reclaiming a majority in the House, experts and polls say.
With control of the House, the Democrats will also control the federal purse springs (spending bills originate in the lower chamber).
But on the eve of the election, the president said in an interview with Sinclair Broadcasting that he wished he had a softer tone during his first two years in office - even as he continued his relentless attacks on political rivals.
Cramer, 57, who has served three terms in the House of Representatives, now leads in the polls. Republicans retained control of the Senate.
O'Connell said "Democrats are going to launch many investigations and launch many subpoenas" if they control the House.
Turnout is traditionally low in the USA mid-terms, with the 2014 election seeing a post-war record low of just 37%.