Democrats had expected a blue wave on election night, hoping to expand their presence in the House of Representatives and even wrest control of the Senate away from Republicans. But the future balance of power in the Senate is still unclear as states continue to count their ballots.
And although Democrats are still expected to keep their control of the House, they lost six incumbents as Republicans eat away their majority.
Limited Democratic influence in Congress could force former vice president Joe Biden to pursue a relatively limited, less ambitious agenda if he beats Donald Trump to the presidency.
Despite raising more campaign funds than Republicans, Democrats flipped only two Senate seats while losing another.
Former Colorado Governor John Hickenlooper ousted Republican incumbent Cory Gardner, while Arizona’s Republican incumbent Martha McSally lost to former astronaut Mark Kelly. However, Alabama’s Democrat incumbent Doug Jones lost to former American football coach Tommy Tuberville.
In Iowa, Republican incumbent Joni Ernst coasted to victory in what was expected to be a close race against Democrat Theresa Greenfield.
South Carolina's Lindsey Graham and John Cornyn of Texas also soundly defeated their opponents despite polls indicating a Democratic groundswell in both states – neither happened. Mr Graham's victory came after his Democratic opponent, Jaime Harrison, raised $57 million between July and September – the largest haul for any Senate race in US history.
Montana's Senate incumbent Steve Daines also fought off a challenge from Steve Bullock, the state's Democratic governor.
And while Mr Biden won Maine, the Republican incumbent Susan Collins defeated Democratic challenger Sara Gideon.
The Path Forward Democrats could still win a Senate majority, but not with the numbers they had hoped to gain. While all eyes are on Michigan to see whether the state will come down in favour of Mr Biden or Mr Trump, another race in that state will help determine the Senate's future balance of power.
The race between Democratic incumbent Gary Peters and Republican challenger John James remains as tight as the state's presidential contest.
As the result of a special election, there are two seats up for grabs in Georgia where Republican incumbents David Perdue and Kelly Loeffler battle Jon Ossoff and Raphael Warnock respectively. Under Georgia election law, a Senate race goes to a run-off election in January if a candidate does not win more than 50 per cent of the vote in either race.
The Loeffler-Warnock race is likely to go to a January run-off due to a third Republican candidate splitting the vote, but Mr Perdue has passed the 50 per cent mark needed to avoid a January election. It remains to be seen whether Mr Ossoff can beat his lead and secure the seat, but most of the uncounted ballots remaining in Georgia are from traditionally Democratic strongholds.
The outcome of the North Carolina is unclear. The Republican incumbent, Thom Tillis, faces Democrat Cal Cunningham and a winner is unlikely to emerge until later this week.
Senate majority leader Mitch McConnell laid out what's on the line in the remaining races for Senate in his victory speech on Tuesday night after securing his seventh term as Kentucky's senior senator. He beat a challenge from Democrat Amy McGrath, whose campaign funds exceeded his by $30m.
Mr McConnell addressed calls from the Democratic base to reform the Senate and US courts should they manage to take control of the Senate. "This is no time to declare war on our institutions because one side is angry that the framers made it hard to achieve radical change," he said.
Some Democrats have called for abolition of the Senate filibuster, which requires 60 votes to get most legislation passed instead of 51, and for expanding the Supreme Court.
Mr McConnell expedited the confirmation of Supreme Court Justice Amy Coney Barrett last month despite refusing to give former president Barack Obama's nominee Merrick Garland a Senate vote in 2016 on the grounds it was an election year.
This tipped the court's balance of power in favour of conservative justices, and prompted calls among many Democratic rank-and-file voters to expand the Supreme Court – a process known as packing.
Mr Biden has vowed to establish a bipartisan committee to study potential court reforms should he win the presidency.
