Democrats held a seat in Delaware, a state that President Joe Biden represented as a senator for 36 years, with a victory for fourth-term Congresswoman Lisa Blunt Rochester.
In Maryland, the Democrats retained a seat vacated by Ben Cardin, with Angela Alsobrooks beating the popular Republican Governor Larry Hogan.
They will become the first black female senators to represent their respective states.
In New Jersey, Democrats retained a seat that was vacated by Bob Menendez, who resigned after he was found guilty of accepting bribes for political favours. Andy Kim, 42, will become the first ever Korean-American senator.
In California, Rep Adam Schiff, who led the first of two impeachment investigations against Trump, retained a seat held by late Democratic Senator Dianne Feinstein.
Other contests that could have swung control of the upper chamber were taking place in Arizona, Pennsylvania, Montana, Michigan, Nevada, Nebraska and Wisconsin.
Both parties are also vying for control of the House, although the balance of power there will not be known for several days.
The vast majority of House elections are happening in “safe districts” – regions where one party is nearly certain to win. But a handful of other races in swing districts could determine who controls Washington DC.
House races that could swing the balance are taking place a wide variety of states, with closely watched elections happening in California, New York, Washington, Maine and Alaska.
