Gallup found that 83% of Republicans predicted that the GOP would keep its Senate majority on Election Day, while only 49% of Democrats believed their party would obtain a majority in the Upper House.
If Democrats capture the House, they could block Mr Trump's agenda and launch congressional investigations into aspects of Mr Trump's administration, from his tax returns to possible business conflicts of interest and the nature of his 2016 campaign's ties to Russian Federation.
Women also contested governor's races across the country.
If Donnelly loses, the Democrats' hope of taking over the US Senate is probably lost, too. Ted Cruz (R-Texas) and Democratic challenger Beto O'Rourke tipping toward Cruz, according to NBC News and the Associated Press. He lauded his party for winning at least half the 10 seats they were defending in states Trump carried and preventing Republicans from capturing a filibuster-proof majority. Republican Brian Kemp, also secretary of state in Georgia, is running the election as well as running against Democrat Stacey Abrams. The GOP's grip on high-profile governorships in Georgia and Wisconsin were at risk as well.
The president spent Election Day calling allies, tweeting endorsements and following news coverage, after concluding a six-day rally blitz in Missouri late Monday. Voters reported waiting up to three hours to vote.
Just 42 percent of registered voters cast ballots in the 2014 congressional midterm elections, the lowest turnout since at least 1964, according to the US Census Bureau.
Almost 40 percent of voters cast their ballots to express opposition to the president, according to AP VoteCast, a national survey of the electorate, while one-in-four said they voted to express support for Trump.
Overall, 6 in 10 voters said the country was headed in the wrong direction, but roughly that same number described the national economy as excellent or good. A recent CNN poll found health care was the top issue for 28% of likely voters in Nevada, followed closely by immigration at 26%.
"The president absolutely is the focal point". Republicans now hold a 51-49 edge in Senate seats and are a strong favourite to retain control of the legislative body.
Republicans had another pickup opportunity in New Jersey, where Democratic Sen. Bob Menendez, who, less than a year ago, stood trial for federal corruption charges.
A similar increase could be seen in certain sectors of the Chinese stock market in the event of a flipping of the House to Democrats.
In Virginia, political newcomer Jennifer Wexton defeated two-term GOP Rep. Barbara Comstock. Trump packed his closing argument with hardline immigration rhetoric and harsh attacks on Democrats as he stared down the prospect of Republican losses that could shadow his presidency.
In southern Oregon, Democrat Jeff Golden, a river guide and public television production manager, edged Republican Jessica Gomez to take Republican Sen.More news: Bryce Harper Won't Be Signing With Yankees
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All 435 seats in the US House were up for re-election, although fewer than 90 were considered competitive while Some 35 Senate seats were in play, as were nearly 40 governorships and the balance of power in virtually every state legislature.
Vermont independent Sen. Bernie Sanders and Democrats Elizabeth Warren of Massachusetts, Kirsten Gillibrand of NY and Minnesota's Amy Klobuchar were easily re-elected.
Sanders spoke to reporters at the White House Tuesday night, as election returns were still coming in. Among those expected: Vice President Mike Pence and former House Speaker Newt Gingrich, an informal adviser to the president.
The political and practical stakes were sky-high.
If the Democrats win control of the House of Representatives, as many pundits expect, it might make it far more hard for the Trump administration to continue along its current policy agenda, and could lead to a possible roll-back of some of the president's more aggressive trade policies. It will also give Trump's opponents more scope to investigate his behaviour. The Senate, where only 35 seats are up for grabs, has a map that favors the Republicans.
Pennsylvania looked particularly daunting for Republicans after court-imposed redistricting and a rash of retirements put several seats in play.
The president has always been accused of stoking hatred as he courts white nationalists in the USA, but his supporters have stood by him as Trump hosted massive rallies in several states in the lead-up to Tuesday's vote. Democrats are relying more upon women, people of color, young people and college graduates.
A Democratic victory in the House would further hinder the border wall plan and complicate congressional approval of a deal to update the North American Free Trade Agreement.
Tuesday's results will be colored by the dramatically different landscapes in the fight for the House and Senate.
The party were most optimistic about the House, a sprawling battlefield set largely in America's suburbs where more educated and affluent voters in both parties have soured on Mr Trump's turbulent presidency, despite the strength of the national economy.
Democrats boast record diversity on the ballot.
Three states could elect their first African-American governors, while several others were running LGBT candidates and Muslims.
The Senate is the smaller chamber, with two senators per state - 100 senators total.