Senator Catherine Cortez Masto's win means the Democrats will control at least 50 seats in the US Senate. Photo: AFP
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.
"I'm not surprised by the turnout. I'm incredibly pleased. And I think it's a reflection of the quality of our candidates," Biden said in Cambodia, where he is attending a summit.
Joe Biden says he's pleased with the results. Photo: JIM WATSON
Schumer said the country "showed that we believed in our democracy and that the roots are strong and it will prevail as long as we fight for it".
Cortez Masto was neck-and-neck with her challenger Adam Laxalt throughout the midterm elections.
The Republican gained notoriety two years ago for championing defeated former president Trump's false claims of election fraud. One recent poll had Laxalt making inroads with Latino voters, who make up one in five eligible voters in Nevada.
But Cortez Masto managed to secure victory, and with it her party's control of the Senate.
The result is a big blow to the Republicans, who were hoping for a "red wave" - an electoral rout which would deliver a harsh rebuke of President Biden and his Democrats.
While the Republicans have made modest gains and remain favoured 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.
Judicial nominations at stake
A Democratic-controlled Senate will provide insurance to Biden that his nominees to fill dozens of federal judgeships will win confirmation under the guidance of Schumer.
That would be particularly crucial to Democrats if a seat on the US Supreme Court, which now has a 6-3 conservative majority, were to open up in the final two years of Biden's term.
When the outgoing Senate returns on Monday for a post-election work session that could run through late December, Schumer aims to immediately confirm two more federal judges awaiting final votes.
Hovering over the 2022 midterm elections all year has been Trump, who used his continued popularity among hard-right conservatives to influence the candidates the Republican Party nominated for congressional, gubernatorial and local races.
With Republicans' lacklustre performance on Tuesday - even if they do win narrow majority control of the House - Trump has been blamed for boosting candidates who were unable to appeal to a broad enough electorate.
Many Donald Trump backed candidates have fared poorly in the mid-term elections. Photo: JOE RAEDLE / AFP
Both Laxalt and Georgia's Walker won Trump's backing. Republican losses in either of these two races could further dampen Trump's popularity as advisers say he considers announcing a third run for the presidency in 2024.
House Republicans, if they pull out a victory, have pledged to try to roll back Biden victories on battling climate change and want to make permanent a series of 2017 tax cuts that will expire. They also have planned investigations into Biden administration activities and probes of the president's son, who has had business dealings with Ukraine and China.
-BBC /Reuters