President Donald Trump has hailed a "big victory" in US mid-term elections after Democrats seized the House of Representatives but Republicans consolidated their grip on the Senate.
The Democratic majority in the lower chamber will be in a position to block Mr Trump's legislative programme.
But control of the Senate ensures Mr Trump can still make key appointments.
The vote was seen as a referendum on the president, even though he is not up for re-election till 2020.
The result confirms a historical trend for the party that is not in the White House to make gains in the mid-terms.
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The Republicans' control of the Senate is arguably more significant, says the BBC's Aleem Maqbool in Washington, because Mr Trump focused his campaigning on Senate races, and candidates he backed clearly benefited.
Mr Trump will now feel that his base has endorsed his actions over the past two years, our correspondent says.
House Democratic leader Nancy Pelosi promised that her party would serve as a counterweight to the White House.
Ms Pelosi - who is set to become speaker, a position she held from 2007 to 2011 - told supporters: "Today is more than about Democrats and Republicans. It's about restoring the Constitution's checks and balances to the Trump administration."
Meanwhile the Florida Senate race is heading for a recount after Republican Rick Scott got 50.21% and incumbent Bill Nelson 49.79% of the vote. A margin of less than half a percentage point automatically triggers a recount.
What difference will the new Congress make?
The Democrats gained more than the 23 seats they needed for a majority in the 435-seat lower chamber.
They could now launch investigations into Mr Trump's administration and business affairs, from tax returns to potential conflicts of interest.
The Democrats could also more effectively block his legislative plans, notably his signature promise to build a wall along the border with Mexico.
In the Senate, Democrats were facing an uphill battle because they were defending 26 races, while just nine Republican seats were up for grabs.
The Republicans are on course to increase their representation from 51 to 54 in the 100-seat Senate upper chamber.
Mr Trump has threatened to retaliate for any Democratic investigations with his own probes in the Senate into alleged "leaks of classified information".
If the Democrats think they are going to waste Taxpayer Money investigating us at the House level, then we will likewise be forced to consider investigating them for all of the leaks of Classified Information, and much else, at the Senate level. Two can play that game!— Donald J. Trump (@realDonaldTrump) November 7, 2018
Who are the new faces in Congress?
Female candidates fared particularly well in an election cycle that had been billed as the Year of the Woman.
Two 29-year-old Democrats - Alexandria Ocasio-Cortez and Abby Finkenauer - are due to be the youngest women ever to win House seats.
Ilhan Omar and Rashida Tlaib are the first Muslim women and Sharice Davids and Debra Haaland the first Native American women to be elected to Congress. All are Democrats.
Ayanna Pressley was elected as Massachusetts' first black congresswoman.
What's happening in governors' races?
Governors - who head the executive branch in state governments - have been chosen in 36 out of 50 states.
In Florida, a progressive Democrat conceded after an ugly battle against Trumpist conservative Ron DeSantis.
But in Georgia, Democrat Stacey Abrams - who was hoping to become the first black female governor in the US - refused to concede as her Republican opponent Brian Kemp took a commanding lead after a bitter campaign.
Democrats also captured governorships in Michigan, Illinois and Kansas.
New York Democratic Governor Andrew Cuomo - sometimes spoken of as a 2020 presidential contender - cruised to a third term.