It will also determine whether Senate Democrats can launch investigations into controversies that have surrounded Trump while he has been in office and before he was elected, including allegations surrounding obstruction of justice, Trump's finances and potential Russian collusion.
But as he touched down in IN for the second leg of his final-day tour Monday, even Trump conceded that the House may slip from his party's grasp.
USA intelligence agencies have concluded the Russian government meddled in the 2016 election with social media posts meant to spread misinformation and sow discord, an accusation denied by Moscow.
"We will have a responsibility to find our common ground where we can, stand our ground where we can't", Pelosi said.
Trump called Pelosi, Senate Republican leader Mitch McConnell, Senate Democratic leader Chuck Schumer and several of the Republican winners.
They ended up reversing abruptly, and Donald Trump was the victor.
Pompeo Allows Sanctions Exception for Iran Port Development
State Department says it is exempting Iran's big port project in Chabahar from sanctions in recognition of its importance to landlocked Afghanistan.
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.
Tuesday night's results mark a comeback for the Democratic Party, which last held control of the House in 2010, and will fundamentally shift the balance of power in Washington.
And in Kansas, Democrat Sharice Davids beat a GOP incumbent to become the first Native American and gay woman elected to the House.
Incumbent Democratic Senator Joe Manchin won a hotly contested race in conservative West Virginia, and conservative Marsha Blackburn held a Senate seat for Republicans.
In Missouri, Republican Josh Hawley also unseated incumbent Democrat Sen.
Some of the campaign's biggest Democratic stars lost. In one result that did not come as a surprise, former Republican presidential nominee Mitt Romney won a Senate seat in Utah. In Virginia, Illinois and Florida, moderate Republicans lost.
Key issues on the minds of voters also favored Democrats.
Earth's ozone layer is healing says UN
Use of man-made chemicals called chlorofluorocarbons (CFCs), which release chlorine and bromine, began eating away at the ozone. Paul Newman, chief Earth scientist at Nasa's Goddard Space Flight Center, described the discovery as " really good news ".
Women voted considerably more in favor of their congressional Democratic candidate - with fewer than 4 in 10 voting for the Republican, according to VoteCast, a nationwide survey of more than 113,000 voters and about 20,000 nonvoters - conducted for The Associated Press by NORC at the University of Chicago.
A CNN/SSRS poll released Monday showed Democrats with a gaping 55% to 42% lead over Republicans among likely voters in a generic congressional ballot.
Democrats turned out to register disapproval of Trump's divisive rhetoric and policies on such issues as immigration and his travel ban targeting several Muslim-majority countries.
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. Political gridlock between the White House and Congress could hinder Trump's pro-business agenda and raise concerns about USA political instability, but investors may have already priced this in. Left-leaning OH "populist" Democrat Sherrod Brown, thought initially to be vulnerable in a state that Trump carried by eight points, was quickly called the victor over Republican congressman and Trump buddy Jim Renacci soon after the polls closed.
Republican gains in the Senate are sure to bolster the party's efforts to ram conservative federal judges through confirmation proceedings during a lame duck session that starts next week, as well as next year.
Trump encouraged voters to view the first nationwide election of his presidency as a referendum on his leadership, pointing proudly to the surging economy at recent rallies.
In the Senate, where Republicans were heavily favored to keep control heading into Tuesday's voting, Republican Mike Braun captured incumbent Joe Donnelly's seat in IN and Republican Kevin Cramer beat incumbent Democratic Senator Heidi Heitkamp in North Dakota.
Driver charged in toddler deaths in Brooklyn crosswalk kills herself
In the past, medics had been dispatched to Bruns' home numerous times following 911 calls that she needed emergency medical help. Her auto drove forward, killing Ruthie Ann Blumenstein's 4-year-old daughter and the 1-year-old son of another parent.
While these were the first two districts to flip, Democratic leaders had appeared positive throughout the day on Tuesday.