Democrats will hold on to control of the US Senate after senator Catherine Cortez Masto won re-election in Nevada, Edison Research projects, handing a major victory to President Joe Biden.
Still, Republicans remain close to winning control of the US House of Representatives as officials continued counting ballots cast in Tuesday's US midterm elections.
Cortez Masto narrowly defeated Republican challenger Adam Laxalt, a former state attorney-general who was endorsed by former president Donald Trump.
With Masto's victory on the heels of Democratic senator Mark Kelly winning re-election in Arizona late on Friday, Democrats will control at least 50 Senate seats, with Vice-President Kamala Harris able to break ties in the 100-member chamber.
"America showed that we believe in our democracy, that the roots of democracy are deep and strong," Senate Majority Leader Chuck Schumer, a Democrat, told a news conference on Saturday.
The Senate is currently split 50-50 between Democrats and Republicans. The new Senate will be sworn in on January 3.
Continued control of the Senate means Democrats will still be able to approve Biden's nominees such as federal judges. Should a vacancy open in the next two years, that would mean any appointees to the Supreme Court.
If Democratic senator Raphael Warnock were to win the December 6 Georgia run-off election against Republican challenger Herschel Walker, that would expand Democrats' majority to 51-49.
That would give Democrats an additional edge in passing the few bills that are able to advance with a simple majority of votes, instead of the 60 needed for most legislation.
It would also dilute the influence of Democratic senators Joe Manchin in West Virginia and Kyrsten Sinema in Arizona, "swing" votes who have blocked or delayed some of Biden's major initiatives, including expansions of some social programs.
It could take several days or more before the outcome of enough House races are known to determine which party will hold the majority in the 435-seat chamber.
Republicans continued to have an edge on Saturday but returns were still flowing in for several races, including many in liberal-leaning California.
Biden, speaking in Cambodia, said his party would be focused on winning the Georgia Senate race but regarding keeping control of the House he said: "It's a stretch."
House Republicans, should they prevail, 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.
Democrats scored a significant upset on Saturday with a projected victory in Washington's 3rd congressional district, where Democrat Marie Gluesenkamp Perez defeated Trump-endorsed Joe Kent.
In Nevada, Democrat Cisco Aguilar won the race to be the state's next secretary of state, the top election official. He defeated Jim Marchant, a Republican who supported Trump's false claim that he lost the 2020 election to Biden because of voter fraud.
Democrats also defeated Republican candidates in Michigan and Arizona who had supported Trump's false claim and sought to be their state's top elections officials.
Democrats received an important boost late on Friday when Kelly, the Democratic senator for Arizona, was projected to hold on to his seat, defeating Republican Blake Masters, who like Laxalt was endorsed by Trump. Masters has not conceded the race.
"We've seen the consequences that come when leaders refuse to accept the truth and focus more on conspiracies of the past than solving the challenges that we face today," Kelly, a former navy pilot and astronaut, said in a short victory speech in Phoenix on Saturday.
No winner was projected yet in the race for Arizona governor, where Democrat Katie Hobbs held a narrow lead over Republican Kari Lake, another pro-Trump election denier.