It's not clear how big their margin of victory will be in the House. Moscow denies meddling and Trump denies any collusion.
That would give the Senate the role of blocking House-passed Democratic initiatives on everything from health care to the potential repeal of tax cuts on the wealthy that the GOP enacted a year ago.
The Democratic Party has gained 26 seats held by the Republicans in the House of Representatives and has won 210 so far, against the Republicans' 191.
Nancy Pelosi said: "a Democratic Congress will be led with transparency and openness".
In her victory speech, the veteran California lawmaker talked up a "bipartisan" Congress where Democrats had a "responsibility to find our common ground when we can, stand our ground where we can".
In the final stretch, Trump hardened his rhetoric on issues that appealed to his conservative core supporters, issuing warnings about a caravan of Latin American migrants headed to the border with Mexico and condemnations of what he called United States liberal "mobs". And only twice in the past eight decades has the president's party picked up House seats in the midterms.
"Maybe you get a ripple, but I certainly don't think that there's a blue wave", she told reporters, pointing to several early Republican wins.
The swing in the House came as Democrats overcame gerrymandered districts in a number of states to win seats that in normal election years were out of their reach.
Three dead, including shooter, at Florida yoga studio
Records show that the charges were eventually dismissed after Beierle followed the conditions of a deferred prosecution agreement. He was charged by police with battery in 2016 after he slapped and grabbed a woman's buttocks at an apartment complex pool.
The President framed the election as a choice between what he has described as Republican law and order and a Democratic "mob" and spoke extensively about immigration and securing the border in the final days of the fall campaign season.
President Trump's relentless focus on immigration as his main campaigning issue was rewarded with victories over Democrat senators in Indiana, North Dakota and Missouri - all states which backed him in 2016 - but it was also blamed for turning off support in suburban areas and among women voters.
For Republicans, the loss of one chamber of Congress was somewhat blunted by a better-than-expected performance in the Senate.
Voter turnout could be the highest for a midterm election in 50 years, experts predicted. Joe Donnelly and Heidi Heitkamp and Democrat Andrew Gillum conceded to GOP's Ron DeSantis for the race for Governor. But one of the Democrats, Lyman Hoffman, joined the Republican leadership.
Actually passing many bills, however, will be hard because the GOP will fall well short of the 60 votes needed to break through Democratic filibusters, procedural delays that kill legislation.
Also up for grabs were the posts of governor in 36 states.
Preliminary exit poll data found that Trump was a factor for nearly two-thirds of House voters.
They may make fresh targets for his divisive rhetoric.
Amid record turnout, South Florida officials prepare for Election Day
More results are expected around 9 p.m.as ballots cast today make their way to the Registrar of Voter's warehouse to be counted. More than 35 percent of registered voters in the state have already voted , taking advantage of mail-in ballots and drop boxes.
Republicans started 2017 - the beginning of the two-year, 30th Legislature - with a majority in the House.
Discussion about the possibility of impeaching the President is also likely to grow louder in a Democratic-controlled House, despite the fact that Democratic congressional leaders have tried to tamp down on speculation that the party might pursue impeachment - at least at this point. Democrats needed to win 11 districts to win back control of the chamber for the first time since 2014.
Republicans entered the night commanding the Senate only narrowly, 51-49. Among those expected were Trump's adult children, White House aides, Republican officials and presidential friends.
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.
Investors often favor Washington gridlock because it preserves the status quo and reduces uncertainty, even though many investors this time around had been hoping for a continuation of the Republican agenda.
Recent months have seen rising internal resistance, with dozens of House Democrats and candidates signalling their desire for change at the top - including Tim Ryan, an OH congressman who challenged Pelosi for the leadership after the 2016 election but fell short.
"Everyone was bracing for any possible, insane scenario to show itself tonight, but it basically looks like the baseline consensus forecast was correct", said Michael Purves, head of equity derivatives strategy at Weeden & Co, in NY.
Ted Cruz, the supreme politician with eyes on the White House
Mr Cruz's actions, especially the government shutdown, alienated many of his Republican colleagues in the Senate. Ted Cruz and his Democratic rival, Rep. "It's the time for choosing".