If Democratic Senator Raphael Warnock were to win the 6 December Georgia runoff election against Republican challenger Herschel Walker, that would expand Democrats' majority to 51-49. That, in turn, would give Democrats an additional edge in passing a limited number of controversial bills that are allowed to advance with a simple majority of votes, instead of the 60 needed for most legislation.
Democratic Senators Joe Manchin in West Virginia and Kyrsten Sinema in Arizona are "swing" votes who have blocked or delayed some of Biden's major initiatives, including expansions of some social programs.
But with 51 Democratic seats in the upcoming Congress, Manchin's and Sinema's influence would be slightly diluted.
It was still unknown which party will hold the majority in the US House of Representatives for the next two years. Republicans continued to have an edge, but returns were still flowing in for several races, including many in liberal-leaning California.
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.
It could take several days or more before the outcome of enough House races are known to determine party control of that 435-seat chamber.
Democrats got an important boost late on Friday when Kelly, the Democratic senator for Arizona, was projected to hold onto his seat, defeating Republican Blake Masters, who like Laxalt was endorsed by Trump. Masters has not conceded the race.
Kelly, a former Navy combat pilot and astronaut, delivered a short victory speech to his supporters in Phoenix on Saturday with his wife, former Democratic Representative Gabby Giffords, at his side. His remarks focused on working in Congress in a bipartisan manner.
Kelly did not mention Masters, but said: "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."
The midterm elections saw many Republican candidates, including Masters, echo former President Trump's false contention that he lost the 2020 election to Biden because of massive voter fraud.
In Nevada, where officials in Clark County were processing ballots that arrived by mail and via drop boxes, county registrar Joe Gloria said no candidates have made fraud allegations to his office.
"We haven't heard anything from any campaign related to fraud or questioning" of the process or results, he said at a news conference.
No winner was projected yet in the race for Arizona governor, where Democrat Katie Hobbs holds a narrow lead over Republican Kari Lake. In the race for Nevada's secretary of state, Democrat Cisco Aguilar defeated Republican Jim Marchant, a noted supporter of Trump's false claim of 2020 voter fraud.
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 Majority Leader Chuck 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' lackluster 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.
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.