Democrats are rapidly losing hope of gaining control of the US Senate after underperforming in key states.
Controlling the Senate would have allowed them to either obstruct or push through the next president's agenda.
The party had high hopes of gaining the four necessary seats in Congress's upper chamber, but many Republican incumbents held their seats.
The Democrats are projected to retain their majority in the lower chamber, the House, but with some key losses.
With many votes still to be counted, the final outcome for both houses may not be known for some time.
Among the disappointments for the Democrats was the fight for the seat in Maine, where Republican incumbent Susan Collins staved off a fierce challenge from Democrat Sara Gideon.
However, the night did see a number of firsts - including the first black openly LGBTQ people ever elected to Congress and the first openly transgender state senator.
The balance of power in the Senate may also change next January. At least one run-off election is due to be held that month in Georgia, since neither candidate has been able to secure more than 50% of votes.
This year's congressional election is running alongside the battle for the White House between Donald Trump and his Democratic challenger Joe Biden.
The Democrats had hoped to flip the Republicans' 53-47 majority in the Senate - giving them the power to obstruct the plans of a second-term President Trump or push through the agenda of a first-term President Biden.
Of the 35 Senate seats up for grabs, 23 were Republican-held and 12 were Democrat.
Senators serve six-year terms, and every two years a third of the seats are up for re-election.
Democrats have not had control of the Senate for six years. If Mr Biden wins the White House, his Vice President Kamala Harris will have the power to break any tied votes.
Who are the winners and losers?
Maine Democrats had high hopes of unseating Susan Collins, the 67-year-old moderate Republican who had been trailing her Democrat rival in the polls for months.
But Sara Gideon, 48, conceded in a call to Ms Collins on Wednesday afternoon.
So far, Democrats have managed a net gain of one seat in the Senate election.
Democratic former governor John Hickenlooper won a key Colorado seat from the Republican incumbent Cory Gardner.
Mr Hickenlooper, who stood for the Democratic nomination for president, was governor of Colorado for two terms from 2011 until last year. His rival was considered particularly vulnerable because of his allegiance to President Trump.
In Arizona, former astronaut Mark Kelly defeated Republican incumbent and former fighter pilot Martha McSally. Mr Kelly earlier said he was "confident that when the votes are counted, we're going to be successful in this mission".
However, Republican Senate Majority Leader Mitch McConnell and Trump ally Lindsey Graham have both been re-elected in their seats of Kentucky and South Carolina respectively.
And in Alabama, Republican candidate Tommy Tuberville took a seat from the Democratic incumbent Doug Jones.
Among the candidates for Senate this year were a pastor, a former pilot and a football coach.
The Democrats had also gone into the election hoping to build on their 232-197 majority in the House of Representatives, but early returns suggested this may not happen after losing at least six incumbents and failing to claim any new Republican seats.
However, there were some notable and historic wins during both the congressional and state elections.
Democrats Ritchie Torres and Mondaire Jones, both in New York, became the first gay black members of Congress after they were elected to the House of Representatives.
Democrats Alexandria Ocasio-Cortez, Ayanna Pressley, Rashida Tlaib and Ilhan Omar secured re-election to the House. Together they have been dubbed "The Squad".
In Oklahoma, the state's sole Democratic member of the House, Kendra Horn, conceded to her Republican challenger Stephanie Bice.
One Republican candidate who won a seat in Congress has previously promoted the far-right QAnon conspiracy theory. Marjorie Taylor Greene, of Georgia, was hailed as a "future Republican star" by President Trump earlier this year.
Delaware Democrat Sarah McBride made history as the country's first ever trans state senator.
The Democrats focused their campaigns on controlling the spread of Covid-19 in the US - which has had the world's highest death toll and number of infections for much of the pandemic.
Republicans, meanwhile, largely campaigned on economic issues.