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 Biden’s agenda.
The result is a big blow to the Republicans, who were hoping for a “red wave” – an electoral rout that would deliver a harsh rebuke of President Biden and his Democrats.
While the Republicans have made modest gains and remain favored to win the House of Representatives, the Democrats have performed much better than expected.
Trump – who continues to insist, falsely, that he won the 2020 presidential election – has been making unsubstantiated claims about the midterms.
“The Democrats are finding all sorts of votes in Nevada and Arizona. What a disgrace that this can be allowed to happen!” he posted on his Truth Social platform on Friday.
Trump is expected to announce that he will run for president again in 2024, but candidates he backed received mixed results in the midterms.
Republican Senator Josh Hawley – who represents Missouri – said after the Senate result that the old party was “dead” and it was time for something new.