WASHINGTON — A day after clinching a narrow hold on the Senate, Democrats began laying plans on Sunday to use their majority as a bulwark for President Biden in Congress should Republicans wrest control of the House, including by confirming his nominees, killing G.O.P. legislation on arrival and promoting their own policies to voters.
Defying political gravity and historical midterm trends that have heavily favored the party not in power, Democrats secured a bare-minimum majority in the Senate on Saturday night with the re-election of Senator Catherine Cortez Masto of Nevada. While their margin of control in the chamber will remain razor thin — and far short of the supermajority needed to pass major legislation — it constitutes a lifeline for Mr. Biden, limiting Republicans’ opportunity to wreak havoc on his agenda or to impeach and remove him or other members of his administration.
If Democrats manage to retain the House — a possibility, albeit a remote one given where uncalled races are currently leaning — it would be a true game changer for Mr. Biden, potentially allowing him to push through even more of his agenda in the second half of his term. But even without that, the Senate gives him a critical foothold.
Democrats will retain the power to unilaterally confirm scores of additional Biden-appointed judges. They will also keep control of the Senate floor, allowing them to ensure that, should Republicans win the House majority, any legislation that could frustrate Mr. Biden’s agenda or make life politically difficult for him and other Democrats never sees the light of day in the other chamber.
Speaking to reporters at a news conference in New York on Sunday, Senator Chuck Schumer, Democrat of New York and the majority leader, challenged Republicans to work with his side to pass more of Mr. Biden’s agenda. He noted they had already done so this year on a few substantial pieces of legislation.
“The Republican Party has a choice,” Mr. Schumer said. “They can continue to let the MAGA hard-right lead them — that’s a path to failure. Or they can work with us on important issues that benefit the American people.”
He called the midterm results “a clarion call by the American people” for Republicans to “stop flirting with autocracy, stop spending your time denying the election and work to get something done.”
Even as Democrats celebrated, Senate Republicans confronted divisions in their ranks that threatened to make their minority status even more problematic, with Senator Mitch McConnell of Kentucky, the minority leader, facing a mini revolt from his right flank. It appeared unlikely to topple him but reflected a bitter round of blame-laying and recriminations that could have lasting implications for the party.
It unfolded as the final Senate balance of power remained up in the air. Key holds in Nevada and Arizona — along with a flip in Pennsylvania — have ensured that Democrats will have at least 50 Senate seats in the new Congress, giving them the same bare majority they now have by dint of Vice President Kamala Harris’s tiebreaking vote.
The outcome of the sole unresolved Senate race — the runoff contest in Georgia on Dec. 6 between Senator Raphael Warnock, a Democrat, and his Republican challenger, Herschel Walker — will determine whether Democrats can expand their majority by an additional seat or if Republicans will hold them to the current 50-50 split.
Mr. Biden, a veteran of the Senate who described himself as “a cockeyed optimist,” enthusiastically opined from Phnom Penh, Cambodia, about how much easier life would be for Democrats if they could gain even one additional seat.
“It’s always better with 51,” the president said. “The bigger the numbers, the better.”
But even a 50-50 split gives Democrats a wide range of powers to defend Mr. Biden and keep Republicans on the defensive.
Senate Democrats will be able to block political messaging bills passed by House Republicans and respond with messages of their own, setting up votes on broadly popular elements of their agenda and highlighting G.O.P. opposition.
“The Democrats should be aggressive in putting Republicans on the defensive, pressing hard on why they are blocking much-needed initiatives to help Americans,” Senator Elizabeth Warren, Democrat of Massachusetts, wrote in a guest essay in The New York Times on Saturday entitled “Democrats Just Held the Senate. Here’s What We Do Next.”
Senate Republicans, by the same token, will be unable to bring up legislation that would squeeze Democrats into taking a tough vote, a dynamic that could embolden the most outspoken hard-right members of the House Republican conference.
Should a Republican-led House make good on lawmakers’ promises to impeach members of the Biden administration — such as the attorney general, the homeland security secretary or the president himself — a Democratic Senate would guarantee that the proceedings would go nowhere.
And Democratic control of the Senate assures that a Biden nominee to the Supreme Court would be considered if a vacancy occurred, a prospect that would be uncertain if Republicans were in the majority.
“President Biden’s judicial appointments have been his most unheralded legacy item, but the project would have been stopped dead in its tracks if Republicans had taken over the Senate,” said Brian Fallon, the executive director of Demand Justice, a progressive judicial advocacy group. “We have the chance to confirm another 100-plus Biden judges in the next two years. These nominees would have never seen the light of day if Arizona or Nevada went differently, but now they will get to serve for life. This is game changing.”
Mr. Fallon also emphasized the importance of winning Georgia and giving Democrats a 51-49 majority.
“Georgia is still highly important,” he said, “because we can confirm even more judges more quickly if we don’t have to deal with all the procedural hurdles that come with a tied Senate.”
Senate Republicans were looking toward that contest while licking their wounds from a weaker-than-expected midterm performance. In a potential threat to Mr. McConnell, a group of Republicans, including Senators Rick Scott of Florida, Josh Hawley of Missouri, Ron Johnson of Wisconsin and Marco Rubio of Florida, were pressing for a delay in leadership elections scheduled for Wednesday to allow more time to assess what went wrong in the election and for Georgia’s runoff to be decided.
“The leadership in the Republican Senate says, ‘No, we’re not going to have a plan, we’re just going to run against how bad the Democrats are,’ and then they actually cave in to the Democrats,” Mr. Scott said on Fox News, referring to Democratic-led bills that passed with Republican support. “And now they want to rush through an election.”
He declined to take “off the table” a potential challenge of Mr. McConnell for the top Republican leadership post.
But most Senate Republicans just wanted to move on. No opponent to Mr. McConnell had emerged as of Sunday, and he was expected to retain his position.
“There is no reason to delay; we should go forward,” said Senator Susan Collins, Republican of Maine. “Mitch deserves gratitude for the huge amount that he raised and put into races like North Carolina and Ohio, where we were not in great shape, as well as Pennsylvania, which, had President Trump not come in for a last-minute rally with the gubernatorial candidate in tow, I think we would have won.”
Senator Tom Cotton, Republican of Arkansas, said he would support Mr. McConnell, noting that “so far, no one’s had the nerve to step forward and challenge” him.
“I think it’s better that we move forward with these elections so we can focus, again, on the Georgia runoff,” Mr. Cotton said.
Other senators and top Republican officials professed astonishment that Mr. Scott, who as the chairman of the National Republican Senatorial Committee presided over the defeat, would push for a change in leadership and consider offering himself as a candidate.
They dismissed the call to wait until Georgia’s results were known, noting that Alaska’s Senate race, where two Republicans were leading in a ranked-choice election, might not be decided for weeks and that a new senator would be coming from Nebraska with the looming departure of Senator Ben Sasse for the presidency of the University of Florida.
They also noted that Mr. McConnell had been very helpful to some of those agitating for a delay. Political action committees affiliated with him spent nearly $40 million on behalf of Mr. Johnson, and Mr. McConnell named Mr. Rubio to be the top Republican on the Intelligence Committee.
Chris Cameron contributed reporting from Washington and Grace Ashford from Old Chatham, N.Y.