Democrats kept control of the Senate late Saturday, repelling Republican efforts to retake the chamber and making it harder for them to thwart President Biden’s agenda. The fate of the House was still uncertain as the GOP struggled to pull together a slim majority there.

Senator Cortez Masto’s victory in Nevada gave Democrats the 50 seats they needed to keep the Senate. Seeking reelection in an economically challenged state that has some of the highest gas prices in the nation, Ms. Cortez Masto was considered the Senate’s most vulnerable member, adding to the frustration of Republicans who were confident she could be defeated.

“We got a lot done and we’ll do a lot more for the American people,” Senate Majority Leader Schumer said Saturday night. “The American people rejected — soundly rejected — the anti-democratic, authoritarian, nasty and divisive direction the MAGA Republicans wanted to take our country.”

With the results in Nevada now decided, Georgia is the only state where both parties are still competing for a Senate seat. Democratic incumbent Raphael Warnock faces GOP challenger Herschel Walker in a December 6 runoff. Alaska’s Senate race has advanced to ranked choice voting, though the seat will stay in Republican hands.

Democratic control of the Senate ensures a smoother process for Biden’s Cabinet appointments and judicial picks, including those for potential Supreme Court openings. The party will also keep control over committees and have the power to conduct investigations or oversight of the Biden administration, and will be able to reject legislation sent over by the House if the GOP wins that chamber.

In Phnom Penh, Cambodia, for the summit of the Association of Southeast Asian Nations, Mr. Biden said of the election results: “I feel good. I’m looking forward to the next couple of years.”

He said winning a 51st seat from the Georgia runoff would be important and allow Democrats to boost their standing on Senate committees.

“It’s just simply better,” Mr. Biden said. “The bigger the number, the better.”

If Democrats manage to pull off a win in the House, it would mean full control of Congress for Democrats — and another chance to advance Mr. Biden’s priorities, which he has said include codifying abortion rights. The party still lacks the 60 votes in the Senate needed to move many kinds of major legislative changes.

Mr. Biden, who called to congratulate Ms. Cortez Masto, said he was still hopeful that Democrats could hold the House. “It’s a stretch,” he acknowledged. “Everything has to fall our way.”

The Senate fight had hinged on a handful of deeply contested seats. Both parties spent tens of millions of dollars in Pennsylvania, Arizona, Nevada and Georgia, the top battlegrounds where Democrats had hoped that Republicans’ decision to nominate untested candidates — many backed by President Trump — would help them defy national headwinds.

Democrats scored a big win in Pennsylvania, where John Fetterman defeated celebrity heart surgeon Dr. Mehmet Oz, who was endorsed by Mr. Trump, to pick up a seat currently held by a Republican. Arizona’s Senator Kelly won reelection by about 5 percentage points.

A closely divided swing state, Nevada is one of the most racially diverse in the nation, a working-class state whose residents have been especially hard-hit by inflation and other economic turmoil. Roughly three-fourths of Nevada voters said the country is headed in the wrong direction, and about half called the economy the most important issue facing the country, according to AP VoteCast, a survey of 2,100 of the state’s voters.

Heading into the midterm election, Republicans focused relentlessly on the economy, a top concern for many voters amid stubborn inflation and high gas and food prices. The GOP also hit Democrats on crime, a message that tapped into anxiety, particularly among the suburban voters who turned away from the party in 2018 and 2020. And they highlighted illegal border crossings, accusing Mr. Biden and other Democrats of failing to protect the country.

But Democrats were buoyed by voters angry about the Supreme Court’s June decision overturning the constitutional right to an abortion. They also portrayed Republicans as too extreme and a threat to democracy, following the January 6, 2021, riots at the U.S. Capitol and Mr. Trump’s false claims — repeated by many GOP candidates — that the 2020 election was stolen from him.

Nationally, VoteCast showed that 7 in 10 voters said the Supreme Court’s decision on Roe v. Wade was an important factor in their midterm decisions. It also showed the reversal was broadly unpopular. And roughly 6 in 10 said they favor a law guaranteeing access to legal abortion nationwide.

Beyond Congress, Democrats won key governors’ races in Wisconsin, Michigan and Pennsylvania — battlegrounds critical to Mr. Biden’s 2020 win over Mr. Trump. Republicans, though, held governors’ mansions in Florida, Texas and Georgia — another battleground state Biden narrowly won two years ago.

Though the midterms failed to deliver Republican romps, Mr. Trump remains a major factor in the national party and plans to announce his third run for the presidency Tuesday at his Mar-a-Lago estate in Florida — setting up a potential rematch for the White House with Mr. Biden.