Firstly, in a broad sense, many see the midterms as a referendum on the performance of the sitting president, so poor performance for the Republicans could be an ominous sign of what's to come in 2020.
Current estimates suggest the Republicans have a 80.9 percent chance of keeping control and the Democrats have a 19.1 percent chance.
Former President Barack Obama - on the campaign trail for the Democratic party - said earlier on Monday that "the character of our country is on the ballot". He added: "The Democrat agenda is a socialist nightmare".
Tuesday's results will be colored by the dramatically different landscapes in the fight for the House and Senate.
Facebook said it blocked 115 accounts for suspected "coordinated inauthentic behavior" linked to foreign groups attempting to interfere in Tuesday's USA midterm elections. He energises elements of the Republican base, who love him, and the Democrats, who hate him, at the same time.
"For the past two years, we've watched this administration attack and undermine our democratic institutions and values".
President Donald Trump landed in IN for a campaign rally and took a low-key posture on the prospect of a Democratic takeover - saying he doesn't care if they try to force the release of his taxes and he'll just work "differently" if they win. However, Trump walked away with the victory against all odds.
AccuWeather Data Scientist and Meteorologist Tim Loftus said: "Weather was found to be, on average, almost 20 percent of the change in voter turnout based on our analysis".
Whether they flip the House or not, Democrats are still expected to gain seats in the chamber, given the favorable political environment for Democrats. Even if they lose the House, if Republicans keep the Senate, they still get to control the approval process for new judges.
That has left many Democrats, particularly those from the party's liberal progressive wing, frustrated.
However, the Democrats are expected to fall short of the two seats they need to win control of the Senate. He hammered Trump and Republicans for repeatedly trying to repeal his signature health-care law while at the same time claiming to support the law's protections for those with pre-existing conditions.
Some Democrats have vowed to force the release of his tax returns.
On the Senate side, Indiana Democrat Joe Donnelly's fate will likely be critical to Senate control.
A Reuters/Ipsos poll of 2,673 Americans conducted between 1 and 28 October found that healthcare was the top issue on the minds of Democratic and independent voters heading into the midterms. Angry voters vote. And angry voters vote in midterms. This will be one of the biggest turnouts we have ever seen in a midterm election and that's great news for democracy. And among white men without college degrees, Republicans led 65% to 30%.
She would be the first woman and first black person elected governor of Georgia, and she would be the first black woman to govern any state in the US. He's a military veteran who prides himself on not being a career politician.
"We can not react to some abstract cybersecurity analysts because we do not know who they are and whether they understand anything about cybersecurity", Dmitry Peskov told reporters.
The political realignment, defined by race, gender and education, could re-shape U.S. politics for a generation. The demographic shifts also reflect each party's closing argument.
By the time Britain wakes up tomorrow morning, we will know the result of the 2018 USA midterm elections.
Rasmussen notes, "Forty-three percent (43%) of voters say the country is headed in the right direction".
Bernie Sanders, the leftist populist who some feel would have had a better chance than Clinton to take on Trump in 2016, lashed out Monday at the president, calling him a "pathological liar".
"He is a sexist, a racist, a homophobe, a xenophobe and a religious bigot".
Voting in Miami, Eloisa Alvarez said this election was "super important".