WASHINGTON — The battle over the Senate filibuster escalated sharply on Tuesday, as President Biden for the first time threw his weight behind changing the rules even as Senator Mitch McConnell, the Republican leader, threatened harsh reprisals if Democrats moved to weaken the procedural tactic.

In an interview with ABC News, Mr. Biden gave his direct endorsement yet of overhauling the filibuster, saying that he preferred a return to what is known as the talking filibuster: the requirement that opponents of legislation occupy the floor and make their case against it.

I dont think that you have to eliminate the filibuster; you need to do it, what it was when I first got to the Senate back in the old days, the president said. “You had to stand up and command the floor, and you had to keep speaking. ” The comments were a substantial departure for Mr. Biden, a 36-year veteran of the Senate who has been frequently described by aides as loath to alter Senate procedure.

Its getting to the point where, you know, democracy is having a hard time functioning, ” he added.

Currently, senators need only to register their objections to legislation to force supporters to produce 60 votes to break the filibuster, which is now a near-daily portion of Senate life. Requiring opponents to hold the floor would put more of the burden on them and theoretically make it tougher for them to sustain their resistance.

Mr. Biden’s comments came as Mr. McConnell issued his stark warning and as the president’s allies on Capitol Hill started building a public case for the elimination of this tactic.

Following Senator Richard J. Durbin of Illinois, the No. 2 Senate Democrat, called for changes to reduce its energy, Mr. McConnell, of Kentucky, bluntly promised a “scorched earth” response and pledged to grind the Senate to a standstill and derail Mr. Biden’s agenda if Democrats took this step.

Everything that Democratic Senates didn’t Presidents Bush and Trump, what the Republican Senate didn’t President Obama, would be childs play compared to this disaster that Democrats would create for their own priorities when if they break the Senate, Mr. McConnell said.

He was referring to the prospect that Democrats might resort to a move known as the nuclear option, with their majority status to induce a change in the Senate rules that allow lawmakers to block action on a bill unless proponents can muster 60 votes to move forward. That would effectively destroy the filibuster, allowing the majority party — currently the Democrats — to muscle through any measure on its own.

Progressives have been agitating for such a change allowing Mr. Biden to maneuver his agenda around Republican obstruction, and a growing number of Democrats are openly considering it. The idea has gained strength following the enactment of Mr. Biden’s $1.9 trillion stimulus measure, which Democrats pushed through the Senate without a single Republican vote under a unique budget process , delivering legislation which has so far been well received by the general public and given Democrats a taste of the possibilities of a postfilibuster world.

Seeking to slow Democrats and get the attention of the White House, Mr. McConnell was adamant that Republicans would tie the Senate into knots in retaliation when they took the step. He made his announcement after Mr. Durbin, a respected veteran of the institution, had said on Monday that it was time to stop allowing the minority party to routinely block legislation by requiring a three-fifths majority to advance most bills. It was the most explicit call yet by a Democratic leader to do it.

Mr. Durbin noted that it was Mr. McConnell who institutionalized the use of the filibuster, which historically was used rarely prior to the Kentuckian was in charge. Mr. Durbin said the procedural weapon was a particularly sore point for him, because it’s has for two decades prevented Democrats from enacting the so-called Dream Act, a popular bipartisan bill he wrote that would create a path to legal status for undocumented immigrants brought into the United States as children. Though it has majority support, it has never been able to clear the 60-vote threshold.

I brought it to the Senate floor on five different occasions, and on five different occasions, it was stopped by the filibuster, Mr. Durbin said on Tuesday.

In his speech on Monday, Mr. Durbin argued that the burden should be shifted to opponents of a given bill to maintain a filibuster as opposed to on fans to produce 60 votes to advance it, a view very similar to the one expressed by Mr. Biden. Senator Joe Manchin III, Democrat of West Virginia and one of the party’s major opponents to ending the filibuster, has also said he would entertain the concept of requiring speaking filibusters.

Democrats say they’re not yet ready to move ahead with any attempt to reevaluate the filibuster rules, and they also lack votes within their own party to do so at the moment. For the time being, activists are urging them to build momentum by following the same strategy that they employed in 2013 before they used the nuclear option to effectively eliminate filibusters against executive and judicial branch nominees.

That year, Harry Reid of Nevada, then the Senate majority leader, lined up a series of three highly regarded judicial nominees for vacancies on a prestigious appeals court to show that Republicans were going to obstruct Obama government nominees no matter how qualified they were. Democrats then brought up the nominees repeatedly for floor votes and failed to break Republican filibusters, a process that eventually persuaded enough senators in their ranks they had no option but to lower the 60-vote threshold for nominees to prevent the Obama administration from being denied its right to seat judges.

Proponents of changing the filibuster state Democrats could now do the same with innovative legislation which has majority support. Included in these are a voting rights measure now starting to make its way through the Senate, immigration law, a gun safety bill, a homosexual and transgender rights bill, a pro-union organizing measure and possibly a large-scale public functions measure. The House has spent the last several weeks pushing many of the measures over unanimous or near-solid Republican opposition; the Dream Act is scheduled for a vote on Thursday.

We will need to build a record that could be passed immediately that is failing on the filibuster, said Adam Jentleson, a former top aide to Mr. Reid through the 2013 showdown who has written a new book that strikes the filibuster, is claiming that it has ruined the Senate and impeded public policy that has broad national support.

Senators seem to be increasingly responsive to appeals to end the filibuster as it now exists as they look ahead to the possibility of weeks of Republican resistance to their agenda.

I think people have only had it,” said Senator Amy Klobuchar, Democrat of Minnesota who directs the Rules Committee, about the blocking tactic that she had endorsed in the past. “I don’t believe we should let an antiquated Senate rule undermine the foundation of our democracy and stop us from making progress. ”

Ms. Klobuchar intends to convene a hearing next week on the broad voting rights bill already passed by the House, and she confessed that it was likely to “be a significant test of the filibuster. ”

In his remarks, Mr. McConnell threatened that Republicans would turn the rules against Democrats and try to make it almost impossible to do anything in the Senate if they proceeded with the shift. He referred to the fact that the chamber operates under arcane rules frequently bypassed through what is known as a unanimous consent agreement where no senator objects. If Democrats plunged ahead to gut the filibuster, he cautioned, Republicans would deny consent even on the most mundane of matters, effectively bogging down the Senate.

Let me say this very clearly for all 99 of my colleagues, Mr. McConnell said. “ nobody serving in this room can even begin — can even begin — to imagine what a completely scorched earth Senate would seem like — none. None of us have served one minute in a Senate that was completely drained of comity, and this is an institution that needs unanimous consent to turn the lights on before noon. ”

Mr. McConnell, who noted that he had resisted aggressive demands by President Donald J. Trump to eliminate the filibuster and ram through Republicans’ agenda, said removing it would signify a transformative shift in government and go far beyond what voters intended in electing Mr. Biden and the evenly divided Senate.

“Does anyone really think the American people were voting for an entirely new system of government by electing Joe Biden into the White House along with a 50-50 Senate? ” he asked. “There was no mandate to completely transform America by the American people on Nov. 3. ”

Ms. Klobuchar disagreed with this assessment, saying that Americans did vote for a new approach and that ditching the filibuster might be required to achieve it.

They voted for somebody who is more moderate for president, but someone who is going to do big things, she said. “They voted for change. ”