Polling experts FiveThirtyEight now project that there is an 86% chance - or six in seven - that Trump will lose control of the House.
USA citizens will vote to select their preferred members of Congress, both for the House of Representatives and for the Senate, as well as state Governors and down-ballot positions such as local representatives and judges.
Sabato's Crystal Ball, a nonpartisan political newsletter run by the University of Virginia Centre for Politics, predicted the Democrats would gain 34 seats in the House, 11 more than the 23 they need to form a majority.
More than 60 percent of eligible voters took part in the 2016 vote, when the presidential race was on the ballot.
Election day - which is a working day and not a holiday in the USA - brought to a close one of the most vitriolic campaigns in U.S. history, with each day further dividing the nation that bears the name United States.
US President Donald Trump has made a last bid for voters to back the Republican party on the eve of the mid-term election. "Now it's like the hottest thing".
Limbaugh said he was assuming the massive early voting turnout would indicate huge crowds of voters on Tuesday.
Neil Sroka, the communications director for Democracy for America, a progressive political action committee, said: "In the era of Donald Trump, it's nearly impossible for anything not to be about Trump: people want a check on this administration".
If the Democrats come up short they face another cycle shut out of power.
Facebook's purge is part of countermeasures to prevent abuses like those used by Russian groups two years ago to sway public opinion ahead of the 2016 U.S. presidential election. "I wouldn't say it's as important as '16, but it's right up there". "I just love the deregulation and I just want to keep everything moving, because I'm loving it".
Republicans control the House and the Senate as well as the presidency, giving them total law-making power in Washington DC.
Democrats have accused Republicans of trying to suppress turnout among racial minorities in states like Georgia, where Republican gubernatorial candidate Brian Kemp is overseeing his own election.More news: Jon Jones Isn't Interested In Trilogy Fight With Daniel Cormier
There are several Democratic House targets, too, especially in NY and MI, but Republicans could pick up two Democratic-held seats in rural Minnesota.
The incumbent Democratic Senator Joe Donnelly is under pressure from his Republican challenger Mike Braun.
Former President Barack Obama has been campaigning with Democratic candidate for Georgia Governor Stacey Abrams.
Results from the West will begin to roll in, and there are key Senate races to watch in Montana and Nevada.
The short version? If the midterms go badly for him, and Trump's party loses control of the House of Representatives, he will be left in a deeply uncomfortable position, unable to pass legislation without help from his political opponents.
All seats are contested every two years. "Today, we say enough".
"We have to have a very clear alternative", she said. She urged voters to "vote against radicalism, bigotry, and corruption" and "for fantastic candidates all over the country - including a historic number of women".
One such race involving a woman is seen as a potential bellwether for how the day may go.
Michael Walz, a former Green Beret commander who served in Afghanistan with Greek roots, is running against Democrat Nancy Soderberg, a former Ambassador to the United Nations during Bill Clinton's administration, for Florida's 6th Congressional District in what has been described as a "toss-up" race.
But as he visited IN on Monday he conceded the House may slip from his party's grasp.
"We'll just have to work a little bit differently", he told reporters.
Under the USA constitution a president can be removed from office "for, and conviction of, treason, bribery, or other high crimes or misdemeanours".