Republicans have controlled the House since the conservative Tea Party movement propelled the GOP to a wave election in November 2010.
Democrats were riding to victory Tuesday with strong support from women turned off by Trump's bullying style and among suburban voters with higher incomes and education levels. Also victorious was Republican Mitt Romney, the vanquished 2012 GOP presidential candidate, who grabbed an open Utah seat.
California Democrats hold 39 of their Party's 193 House seats, or over 20 percent.
Democratic victories included the defeat of 11-term Texas Representative Pete Session, the chairman of the Rules Committee, by former professional football player Colin Allred in a Dallas-area district.
In gubernatorial races, Republican Kris Kobach, a Trump ally, was beaten by a Democrat in Kansas.
Sixty of those vulnerable districts are now under Republican control or were left vacant when a Republican lawmaker resigned. One of the most widely watched races, between progressive Democrat Andrew Gillum and Republican Ron DeSantis for governor, is a virtual tie.
"I'd love to get along and I think after the election a lot of things can happen", Trump said.
The O'Rourke-Cruz race is one of a half-dozen Senate contests that are being closely watched to determine control of the chamber.
Another election observer, the Center For Politics at the University of Virginia, is predicting Democrats will gain at least 30 seats on Tuesday.
Sen. Bob Menendez of New Jersey beat Republican challenger Bob Hugin to win a third term.
Republican Mike Braun opened an early lead in the crucial Senate showdown in IN with incumbent Democrat Joe Donnelly, but the race was too close to call with just 20 per cent of the votes in.
In Virginia's 7th District, a Republican-leaning area near Richmond, Republican Rep. Dave Brat was in a tight race with Democrat Abigail Spanberger, a former Central Intelligence Agency officer.
But with his party losing its majority in the House, the results represented a bitter setback for Trump after a campaign that became a referendum on his leadership.
Romney clinched the win Tuesday as he defeated Democrat Jenny Wilson, a member of the Salt Lake County council.
Despite not being on the ballot, Trump made the midterm elections about him.
"This was a hard campaign", Curbelo told reporters in Miami. "I just couldn't believe it", he said, referring to Senate hearings were held where Kavanaugh was accused of sexually assaulting a woman when they were both teenagers. While the Republican president is not on the ballot, the campaign has been indeed largely about him, with Democrats missing no opportunity to tear into the administration's immigration policies, branding the president racist and even comparing him to Osama bin Laden.
In one example, voter turnout in a critical swing district in Florida was 87% of what it was in 2016, when Trump won the presidency.
Democratic candidates for the House flipped seats in several suburban districts outside of Washington, Philadelphia, Miami, Chicago and Denver that were considered prime targets for turnover because they were won by Hillary Clinton in 2016.
"The midterm elections used to be, like, boring", Trump told a crowd in Cleveland, Ohio, on Monday.
Almost 40 percent of voters cast their ballots to express opposition to the president, according to AP VoteCast, the national survey of the electorate, while one-in-four said they voted to express support for Trump. His supporters were also very motivated to head to the polls to ensure Trump can continue to make the changes they want for America.
"This is the year of the woman, and the fact that women were willing to put themselves on the line is important, whether they've been Republicans or Democrats", said Shalala, a first-time candidate for elected office.
If ever there was an off-year election for younger voters to break tradition, this is it.
The White House and congressional leaders condemned the incidents, but the events have sparked questions and recriminations over whether heated political rhetoric contributed to the violence. He was active on Twitter throughout much of the day, blasting out endorsements for Republican candidates and weighing in on the balloting.
Many Democrats, already benefiting from anti-Trump enthusiasm, focused on bread-and-butter issues like maintaining health insurance protections for people with pre-existing medical conditions and safeguarding the Social Security retirement and Medicare healthcare programmes for senior citizens. Under President Trump, over 4 million new jobs have been created, unemployment is the lowest it has been since 1969, wages have increased, taxes have decreased, unneeded regulations have been slashed, the median household income is up to a post-recession high of over $61,000, consumer confidence is at an 18-year high, over $450 billion dollars has been poured into the United States and the economy is on pace to grow at over 3 percent in 2018 for the first time since 2005.