Sen. Catherine Cortez Masto, D-N.V., on Sunday after her projected win against Adam Laxalt. David Becker/The Washington Post

WASHINGTON — Democrats on Sunday were celebrating retaining their majority in the Senate after Sen. Catherine Cortez Masto (D-Nev.) was projected to win reelection, even as control of the House remained undetermined. Her victory secured the 50th Senate seat for Democrats and quashed Republicans’ hopes of taking control of both chambers of Congress, as many had predicted in the weeks leading up to Election Day.

“When far-right Republicans said they knew better, I knew we would prove them wrong,” Cortez Masto said in a victory speech Sunday. “This election, Nevadans rejected the far-right politicians working to divide us. We rejected their conspiracies, their attacks on our workers, and their efforts to restrict our freedoms.”

With the Senate runoff in Georgia next month between Sen. Raphael Warnock (D) and Republican Herschel Walker, Democrats have a chance to pick up a 51st seat, a stunning feat in a midterm election year that typically does not favor the party in power.

The developments have prompted some frustrated Republicans to call on Senate Minority Leader Mitch McConnell (R-Ky.) to clearly lay out a Republican agenda and to delay their leadership elections that are scheduled for Wednesday.

If Democrats win a 51st Senate seat through the Georgia runoff election, Senate Majority Leader Chuck Schumer (D-N.Y.) would be able to bypass a power-sharing resolution with Republicans, in place because of the 50-50 Senate, and more easily confirm President Joe Biden’s judicial nominees.

Senate Majority Leader Chuck Schumer, D-N.Y. Hans Pennink/Pool via AP

Democrats now have more leeway to use their narrow congressional majorities to push through final priorities before they may lose control of the House in the new year. If Republicans had won back the Senate, Democrats would have spent much of the time frantically confirming Biden’s judicial picks before losing the gavels of Senate committees.

Schumer on Saturday night called the results a “vindication” for the Democratic agenda and a rejection of Republican extremism. On Sunday, he called on Republican lawmakers to work with Democrats but declined to get into specifics about what they would try to accomplish. He said Democrats would try to have “as productive a lame-duck session as possible.”

“Maybe the Republican Party, which has been so negative on so many different issues, will realize that the election was a clarion call by the American people: Stop all this negativity, stop flirting with autocracy, stop spending your time denying the election, and work to get something done,” Schumer said.

Some Senate Republicans, including Rick Scott (Fla.), chair of the chamber’s Republican campaign arm, criticized McConnell for allowing Republicans to cross the aisle and help Democrats pass some key pieces of legislation with their slim majority over the past two years, including an infrastructure bill and investment in manufacturing of microchips.

“Republican leadership caved in on the debt ceiling, caved in on the gun deal, caved in on a fake infrastructure deal,” Scott said Sunday on Fox’s “Sunday Morning Futures,” while criticizing McConnell for not releasing his own Republican agenda ahead of the midterms.

Scott’s Republican agenda, which calls for tax increases on lower-income people and frequent reauthorization votes for Social Security and Medicare, has been blamed by some Republicans, including New Hampshire Gov. Chris Sununu (R), for scaring older voters ahead of the midterm elections. The White House and Democrats used the plan repeatedly to warn that Republicans wanted to target entitlement programs if elected.

Rep. Chris Pappas (D-N.H.) clinched a third term – a feat that hasn’t happened in his district in 25 years – because he hammered Trump acolyte Karoline Leavitt for her stances on Social Security.

Sen. Ron Johnson (R-Wis.), who narrowly won reelection, echoed much of Scott’s criticism on Fox News and also called for a delay in the leadership elections. But other Republican senators, including Sens. Tom Cotton (Ark.) and Bill Cassidy (La.), defended McConnell publicly Sunday and said they supported his leadership bid.

No senator has yet announced a challenge to McConnell, who will become the longest-serving leader of either party in the Senate in the next Congress if he is reelected. A representative for McConnell declined to comment Sunday.

Sen. Elizabeth Warren, D-Mass. Tom Williams/Pool via AP

Others cast the blame squarely on former president Donald Trump, who backed several Republican candidates in battleground states who lost, including Pennsylvania Senate candidate Mehmet Oz and Arizona Senate candidate Blake Masters.

“It’s basically the third election in a row that Donald Trump has cost us the race. And it’s like, three strikes you’re out,” Maryland Gov. Larry Hogan (R), a longtime Trump critic, said on CNN’s “State of the Union.” “The definition of insanity is doing the same thing over and over again expecting a different result,” he said. “I’m tired of losing. That’s all he’s done.”

Sen. Elizabeth Warren (D-Mass.) on NBC’s “Meet the Press” on Sunday credited Biden and Democratic voters for the midterm wins. She called on her party to lift the debt ceiling in the lame duck session to ensure Republicans are not able to withhold their votes for lifting it next year if Republicans win back the House. Some House Republicans had floated the idea of demanding spending cuts and repealing IRS funding in return for lifting the debt ceiling.

House Republican aides, who spoke on the condition of anonymity to discuss private deliberations, conceded that they will need Democrats’ help if they secure a slim majority to approve must-pass legislation like funding the government. Many are hoping Democrats pass the debt ceiling in the lame-duck session, a concession from just weeks ago when they were hoping to use that as leverage to pass core GOP priorities next year.

Schumer said he would discuss lifting the debt ceiling in the lame-duck session with his members. It’s unclear if enough Senate Republicans would cross the aisle to back the move, or if even the entire Democratic caucus would be willing to preemptively lift the debt ceiling on a party-line vote.

Control of the House remained in the balance on Sunday, with neither party yet securing the 218 seats required to take the majority. Most of the uncalled congressional races are in California, where ballots are valid as long as they are postmarked by Election Day and where final election tallies could take weeks to determine.

In a major upset Saturday night, Democrats flipped a seat in a reliably Republican district in Washington state as Democrat Marie Gluesenkamp Pérez beat Trump-backed Republican challenger Joe Kent. The former president promoted Kent in the primary over Rep. Jamie Herrera Beutler, who voted to impeach Trump after the Jan. 6, 2021, attack on the Capitol.

As of Sunday afternoon, Republicans have won 211 House seats, while Democrats have won 203. Democrats have a slim chance of retaining control of the House, though Republicans are still favored to take a narrow majority. Despite that, House Speaker Nancy Pelosi (D-Calif.) struck an optimistic and celebratory tone in interviews Sunday.

“It was not anything that we ever accepted when the pundits in Washington said we couldn’t win because history, history, history. Elections are about the future,” Pelosi said on ABC’s “This Week.” “I’m very proud of our candidates, both our incumbents as well as our red-to-blue candidates. They never accepted the punditry that they couldn’t win, they had courage, they had purpose, and they understood their district.”

Pelosi waved off any questions about whether she would run for House speaker again, if Democrats hang on to the majority in the chamber, saying she only wanted to focus on race results. She added that she was “disappointed” with what happened in New York.

That was where Rep. Sean Patrick Maloney, chairman of the Democratic Congressional Campaign Committee, lost his race. “But we haven’t given up,” Pelosi said. “We have the White House, we have the Senate, and we’re going to have a big strong vote in the House, a very different outcome than some would have predicted.”

The Washington Post’s Steven Zeitchik contributed to this report.