But with Mr Trump's first two years in office dividing opinion across the country, Democrats see these elections as a chance to take back control of both chambers of Congress from the Republicans.
Democrats have picked up at least 23 House seats, putting them on track to reach the 218 needed to seize control from Republicans after eight years.
Trump was expected to address the results in greater detail at a White House news conference later Wednesday.
Republicans still had advantages in some areas, giving them hope of retaining a slim majority.
Republicans retained control of the Senate by winning a special election to preserve their one-vote majority and ensuring that divided government will continue in Minnesota. It wasn't clear what "leaks" he was referring to.
Districts around the country reported unusually high turnout: according to Michael McDonald of the US Elections Project, 38.4 million Americans cast their ballots early ahead of this election, compared with 27.4 million in the 2014 midterm.
Anticipating the possibility of keeping the Senate but losing the House, aides in recent days had laid out the political reality to Trump, who could face an onslaught of Democratic-run investigations. The Republican incumbent had been branded Barbara "Trumpstock" by Democrats in a race that pointed to Trump's unpopularity among college-educated women in the suburbs.
In suburban areas where key House races were decided, female voters skewed significantly toward Democrats by a almost 10-point margin.
One key race in Georgia for the state's governorship has attracted much attentions and visits from Mr Trump, his vice-president Mike Pence and, on the Democrat side, former president Barack Obama and talk show host Oprah Winfrey.
The results were more mixed deeper into Trump country.
And in Kansas, Democrat Sharice Davids beat a GOP incumbent to become the first Native American and gay woman elected to the House.
Democrats failed to defeat a vulnerable incumbent in Kentucky, where Republican Rep. Andy Barr won over former Marine fighter pilot Amy McGrath.
United States stocks opened flat on Tuesday as investors awaited the election results.
The elections will mark the first nationwide vote since Russian Federation targeted state election systems in the 2016 U.S. presidential race but federal, state and local officials have sought to reassure the public that their voting systems are secure.
Democrats' dreams of the Senate majority, always unlikely, were shattered after losses in top Senate battlegrounds: Indiana, Missouri, Tennessee, North Dakota and Texas.
Some hurt worse than others.
An open seat that included Atlantic City was also ripe for Democratic pickup by state lawmaker Jeff Van Drew after the GOP campaign committee abandoned Republican Seth Grossman over racially charged comments.
"I like the fact that he is not a politician, and I forgive some of the socially incorrect or politically incorrect things that he says".
"It's all fragile", he said on the call.
But what are the midterm elections and why are they important? Milner, who said he backed Republican congressional candidates, added, "It's like, 'OK, no matter what Trump wants to do, we're not going to vote for it.' I don't like that". Several television networks, including the president's favorite Fox News Channel, yanked a Trump campaign advertisement off the air on the eve of the election, determining that its portrayal of a murderous immigrant went too far.
Kobach had built a national profile as an advocate of tough immigration policies and strict voter photo ID laws. Voters scored Trump positively on the economy and for standing up "for what he believes in".
The president's current job approval, set at 40 percent by Gallup, was the lowest at this point of any first-term president in the modern era.
Anxious Republicans privately expressed confidence in their narrow Senate majority but feared the House could slip away. It's called the midterm penalty for the president's party.
Vermont independent Sen. Bernie Sanders and Democrats Elizabeth Warren of Massachusetts, Kirsten Gillibrand of NY and Minnesota's Amy Klobuchar were easily re-elected. A handful of others played outsized roles in their parties' midterm campaigns, though not as candidates, and were reluctant to telegraph their 2020 intentions before the 2018 fight was decided.
At the same time, Democrats re-elected embattled New Jersey Sen. Cory Booker, California Sen.