The Democrats will retain majority control of the US Senate after winning a pivotal race in the state of Nevada.
Senator Catherine Cortez Masto is projected to defeat Republican challenger Adam Laxalt, who was backed by former President Donald Trump.
The results amount to the best midterm performance for a sitting party in 20 years.
US President Joe Biden said he was incredibly pleased, and it was time for Republicans to decide “who they are”.
Democratic Senate Majority leader, Chuck Schumer, said the results showed the American people had rejected what he called the “violent rhetoric” of the Republican Party.
The Democrats will now have 50 Senate seats, with Republicans currently on 49.
The remaining seat, Georgia, is going to a run-off in December. In the event of the Senate being divided equally between the two parties, Vice-President Kamala Harris has the casting vote.
Republicans could still take control of the US House of Representatives as votes continue to be tallied from a handful of districts after Tuesday’s elections.
If the Republicans win the House they could still thwart much of Mr Biden’s agenda.
