Tuesday's elections also tested the strength of a Trump-era political realignment defined by evolving divisions among voters by race, gender, and especially education.
Republicans are expected to retain their slight majority in the US Senate, now at two seats, which would let them retain the power to approve US Supreme Court and other judicial nominations on straight party-line votes.
"Tomorrow will be a new day in America", House Minority Leader Nancy Pelosi said at a watch party in Washington.
But the Democrats' edge is narrow.
Quoting sources, CNN reported that the Democrats have already won 141 seats out of a total 218 seats in the House.
What it would mean: This would be a huge win for Democrats, as they'd be able to gum up Trump's agenda and begin to investigate his administration, something the GOP has not done very much of.
FOX News is projecting that the Democrats will retake the U.S. House, but the GOP will retain control of the Senate. It wasn't clear what "leaks" he was referring to. Orrin Hatch. Virginia Democrat Tim Kaine, his party's defeated 2016 vice presidential candidate, won re-election to the Senate.
Fox was alone among US media outlets with the call. Democrats are relying more upon women, people of colour, young people and college graduates.
Record diversity on the ballot may have helped drive turnout.More news: Uganda is vaccinating for fear of Ebola virus spread
How it would happen: A record turnout is expected Tuesday - perhaps higher than any time in the past 50 years for a midterm - but, as in 2016, Trump voters would have to dominate.
Democrats welcomed other firsts, including two Muslim-American women, Rhasida Tlaib of MI and Minnesota's Ilhan Oman, who is also the first Somali-American elected to Congress.
Ms Abrams would be the first black woman elected governor in any USA state and can also count another former president and former governor of Georgia, Jimmy Carter, as a supporter.
Sixty of those vulnerable districts are now under Republican control or were left vacant when a Republican lawmaker resigned. At least three other red districts flipped to blue.
Trump, the GOP's chief messenger, warned that significant Democratic victories would trigger devastating consequences. While, Colorado Democrat Jared Polis has been elected as the first openly gay US Governor while, Kansas Democrat Sharice Davids became first Native American woman in Congress. But in Kentucky, one of the top Democratic recruits, retired Marine fighter pilot Amy McGrath, lost her bid to oust to three-term Rep. Andy Barr.
Tuesday's midterm elections could see the Democrats take control of the House of Representatives.
Republicans paved their path to victory by defeating Democrats Joe Donnelly of IN and Heidi Heitkamp of North Dakota.
Some hurt worse than others.
"Things are pretty good, yet we have all this division and we have this president who's relatively unpopular, so we have this odd juxtaposition", said Kent State politics professor Michael Ensley, citing Trump's poor approval ratings despite a rollicking USA economy and the absence of any major foreign-policy challenges.More news: David Haye Exclusive: How to prepare against a fighter like Usyk
Almost 40% of voters cast their ballots to express opposition to the president, according to a national survey of the electorate, while one-in-four said they voted to express support for Mr Trump.
"It's all fragile", he said on the call.
"Even though I'm not on the ballot, in a certain way I am on the ballot", Mr Trump said during a tele-town hall meeting organised by his re-election campaign on Monday to encourage Republicans to get out and vote.
He bet big on a xenophobic closing message, warning of an immigrant "invasion" that promised to spread violent crime and drugs across the nation. The party controls both chambers of the state legislature and the governor's office in 25 states, with Democrats holding that trifecta in eight states, the group said.
A total of 155 Republican incumbents are defending House seats in districts Trump won by double digits in 2016.
Kobach had built a national profile as an advocate of tough immigration policies and strict voter photo ID laws.
While an August poll by Rasmussen Report indicated Trump's approval rating among African Americans doubled year-on-year to reach 36 percent, most analysts still believe African Americans belong to the Democratic camp. She said she used to vote for candidates from both major parties, but vowed never to support a Democrat because she was so upset by how new Supreme Court Justice Brett Kavanaugh was treated in his confirmation process.
Meanwhile, the close of the 2018 midterm season marked the unofficial opening of the next presidential contest.
Vermont independent Sen. Bernie Sanders and Democrats Elizabeth Warren of Massachusetts, Kirsten Gillibrand of NY and Minnesota's Amy Klobuchar were easily re-elected.More news: Tesla names Robyn Denholm as Chairman to replace Elon Musk
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