Results are also trickling in for US Senate seats, such as for democratic socialist and former presidential candidate Bernie Sanders, who won in Vermont.
In House races, Democrats scored some early victories Tuesday in their quest to wrest control of the chamber, but Republicans were holding on in some key races where they faced strong challengers as initial returns were tallied. That will give Trump a partial claim of victory and allow him to continue his drive to reshape the federal courts.
Despite a seemingly energized grassroots electorate, the Democratic Party has been viewed as a long shot to take back control of the Senate where Republicans now hold a narrow majority of 51 seats to 49 seats for Democrats. Indeed, more than 100 women could be elected in the House for the first time.
That's because a Democratic house might stifle some of President Trump's more extreme policies, particularly those on trade. The party defeated Republicans from the Northeast to the South and across the Midwest.
Opposition control of the house could seriously impede Trump's plans for the next two years, which include trying to build a wall along the Mexican border, possible new tax cuts and more attempts to dismantle Obamacare. 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.
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.
"If the Democrats really believe that this President is controlled by Russian Federation, then why do they say nothing about it?" It is also similar to the ratings for former President Obama in the 2010 midterm election, in which Republicans captured majority control from Democrats.
Sixty of those vulnerable districts are now under Republican control or were left vacant when a Republican lawmaker resigned.
In Texas, Sen Ted Cruz staved off a tough challenge from Democrat Beto O'Rourke, whose record-smashing fundraising and celebrity have set off buzz he could be a credible 2020 White House contender. He said since Trump has been president, he 's hired two more employees for his six-person company. And Florida has a competitive US Senate election, where Democratic incumbent Bill Nelson has led Republican Governor Rick Scott by a few points.
In Virginia's 7th District, a Republican-leaning area near Richmond, Republican Representative Dave Brat was trailing Democrat Abigail Spanberger, a former Central Intelligence Agency officer.
As voters head to the polls Tuesday, nothing was certain. When the Cook Report classified Menendez's race as a toss-up recently, things got tense for Democrats over a race that under normal circumstances they would have won going away.
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. "I feel confident we will win", House Minority Leader Nancy Pelosi said on Tuesday, decked out in a bright blue dress at her party's headquarters in Washington, D.C.
Mid-term elections are often seen as a referendum on the president and there is no shortage of opinions this time around about the current occupant of the White House.
According to CNN, exit polling showed that two thirds of voters considered the election to be about Trump, whose approval rating remains below 50 per cent. Almost 4 in 10 said they were casting ballots to express opposition to him, while just 1 in 4 said their vote was an expression of support.
Today's elections are the first opportunity for Americans to have a say about Donald Trump's presidency. Still, Republican voters tended to be overwhelmingly supportive of the president. With so much at stake, Trump campaigned in over a dozen states with Senate elections since Labor Day, visiting some multiple times.
"An American voter is not happy with one party being in charge", he said.
A majority of the 113,000 voters surveyed said the country is headed in the wrong direction.
Trump and Republicans appeared to have the economy on their side heading into the election: Unemployment in October held at a 48-year low of 3.7 percent.
His opponents say taking control of the House and Senate is the only way to stop Trump's agenda on issues like immigration, nominating judges, and healthcare.
But the Republicans could lose up to 10 state gubernatorial races.
The campaign will be the most expensive midterm in history, projected by the nonpartisan Center for Responsive Politics to exceed US$5 billion.