With Donald Trump elected as the next president, the Republicans retained their majorities in the House and Senate.
Republican dominance over Congress in principle enables Mr Trump to turn his policy plans into law.
But how easily this will happen is unclear given that key party leaders had refused to back him.
The night began with majority control of the Senate up for grabs, with 34 of the 100 seats available.
But the Democrats have so far gained just one seat in the Senate, with Tammy Duckworth, a double amputee Iraq war veteran, taking Mark Kirk's place in Illinois. During a TV debate last month Mr Kirk mocked Ms Duckworth's Thai heritage, but later apologised.
Another bright spot was in Nevada, which Cortez Masto retained for the Democrats, beating Republican Joe Heck to become the first Latina US senator.
The final Senate seat to be declared is New Hampshire, where the result is said to be very close and where Democrat Maggie Hassan has declared victory over incumbent Republican Senator Kelly Ayotte.
Elsewhere Republicans hung on to other key seats:
Democrats also failed to significantly dent Republican advantage in the House, with just five Republican incumbents losing.
House Speaker Paul Ryan, who had distanced himself from Mr Trump after previously endorsing him, won re-election to the House of Representatives in Wisconsin.
"We are eager to work hand-in-hand with the new administration to advance an agenda to improve the lives of the American people." Mr Ryan told supporters.
Ilhan Omar became the US' first Somali-American legislator, with victory in a House race in Minnesota. She came to the US while still a child, escaping Somalia's civil war with her family and spending four years in a Kenyan refugee camp.
And in Nevada, Catherine Cortez Masto, the former state attorney general, was elected as the first Latina US senator.
In other developments, the controversial Republican sheriff of Maricopa County, Arizona, was voted out after he was charged with contempt for ignoring a judge's order to stop patrols targeting Latinos.
The Republican Party has kept its hold over Congress, capping a dire night for the Democrats.