The Democratic majority in the House will be in a position to block President Trump’s legislative program.
However, control of the Senate ensures President Trump can still make key appointments.
The vote was seen as a referendum on Donald Trump, even though the president 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.
House Democratic leader Nancy Pelosi promised that her party would serve as a counterweight to the White House.
Nancy 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.
The Democrats gained more than the 23 seats they needed for a majority in the 435-seat House of Representatives.
They could now launch investigations into President Trump’s administration and business affairs, from tax returns to potential conflicts of interest.
They 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.
President Trump has threatened to retaliate for any Democratic investigations with his own probes in the Senate into alleged “leaks of classified information”.
He tweeted: “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!”
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