US President Donald Trump has hailed "tremendous success" in the mid-term elections after a night of mixed results for his Republican party
But Republicans consolidated their grip on the Senate, ensuring the president can still make key appointments.
Mr Trump said the outcome "defied history" as the ruling party does not usually gain seats in the mid-terms.
At a feisty news conference on Wednesday, Mr Trump offered an olive branch to Democrats, proposing both parties work together on joint legislative priorities.
But he said that if Democratic-controlled congressional committees started serving legal writs against his administration, Republicans would do likewise and gridlock would ensue.
In ill-tempered exchanges, Mr Trump called a CNN correspondent "a rude, terrible person" and told an NBC reporter: "I'm not a big fan of yours either."
House Democratic leader Nancy Pelosi has promised her party will serve as a counterweight to the White House.
Ms Pelosi - who is favourite to become speaker, a position she held from 2007-11 - told supporters: "Today is more than about Democrats and Republicans. It's about restoring the Constitution's checks and balances to the Trump administration."
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"