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.
“I’m not surprised by the turnout. I’m incredibly pleased. And I think it’s a reflection of the quality of our candidates,” Mr Biden said in Cambodia, where he is attending a summit.
Mr 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”.
Ms 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 Mr Laxalt making inroads with Latino voters, who make up one in five eligible voters in Nevada.
But Ms 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.
Mr 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.
Mr 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.
The Republican midterm flameout is now official. Democrats have retained control of the US Senate, which will pave the way for Joe Biden to spend two more years filling the federal courts with his nominees and staffing his administration largely the way he sees fit.
The Georgia Senate run-off is no longer a pivotal contest to determine control of the chamber, although a victory for Democrats there would make holding the majority in two years easier, when the party will have more at-risk seats to defend.
There is still a likelihood, although not certainty, that the Republicans will control a slim majority in the House of Representatives, bringing a variety of headaches for the president.
His legislative agenda is dead, and more aggressive Republican oversight is in store, but even that has a silver lining – if his political opponents are unable to effectively govern due to internal discord.
The consequences of this history-defying midterm election result are still being revealed.
Donald Trump’s political future has been damaged, although how enduringly remains to be seen. Joe Biden’s standing within his party has been bolstered. The political world in the US looks considerably different than it did just a week ago.
The midterm elections are for Congress, which is made up of two parts – the House of Representatives and the Senate.
Congress makes nationwide laws. The House decides which laws are voted on while the Senate can block or approve them, confirm appointments made by the president and, more rarely, conduct any investigations against him.
These votes are held every two years and when they fall in the middle of the president’s four-year term of office, they are called the midterm elections.
Each state has two senators who sit for six-year terms. Representatives serve for two years, and represent smaller districts.
All the seats in the House of Representatives were up for election in the midterms, alongside one-third of the Senate.
Several major states also have elections for their governor and local officials.